This is a topic that I have always been befuddled and confused about.
I have always struggled to understand, and I probably never will fully understand women's preoccupation with confidence. I agree that sexy is not the same as good looking. There I many women I find absolutely beautiful, but I don't find them particularly sexy, while there may be another girl who has more homely features who seem to be just seething with sexuality. Self confidence can be sexy in a woman, but I don't think it is required. I think self confidence must be a more masculine feature, which may be part of the answer.
I mean if a guy is confident, so what? What does that prove? Unless he has actual reasons to be confident, it seems to me that his confidence is hollow. But women seem to be like confidence for its own sake. I can see that confidence allows a guy to be more relaxed and be himself, let his true personality come through, which is a good thing. But what about things like arrogance, swagger, and overconfidence? Are these sexy also? Swagger to me suggests insecurity, because it hints at overcompensating.
Even as a young man, I was always told to be confident, so I've always faked it if I didn't feel it, but I've never fully understood why, and probably never will. Why do women find confidence so attractive?
I know that if a guy is confident, then that allows him to be the aggressor, and that way the woman doesn't have to do the "work" of creating the hookup. And it may hint at hidden positive attributes, but if there are no actual attributes to be confident about, why is it attractive? Or is it? Is it a dominance thing?
This is long. I apologize in advance.
TLDR in bold.
I will try and explain it. It isn't just confidence. When I am talking about confidence it's not confident about just anything.
Confidence in a sexual way is sexy. Confidence in one's masculinity and all that embodies (sexual desire.) Confidence in an "I might be bad news" way is sexy. Confident in an "I am trouble but I just can't help it" way is sexy. Confident in an "I know I am a great fvck" kind of way. It is about being empowered in your OWN sexual nature. Be genuinely unapologetic about it. If you are playful it will take on a playful varnish, if you are reserved it will take on a mysterious varnish, if you are gregarious it will take on a charming varnish...This isn't confidence about being great at your job or being confident about your looks.
It's being confident that you are desirable and it is not apologizing for that self view.
One of the best real life examples I can think of is Benicio Del Toro. That man oozes sexuality. He's been in commercials where he is intentionally mistaken for Antonio Banderas and his line is simply "I am NOT Antonio Banderas" but he doesn't say who he is. He doesn't need to. He gives off a sexual no give a sh1t attitude which is at once intense and completely laid back. His attitude is utterly unapologetic, yet when he is interviewed he is actually quite gracious. He knows he's IT so much he's chill about it. It is what it is. Joe Namath is another example that comes to mind. Same attitude. There is a great ESPN 30 for 30 or A Football Life (don't recall which) on Namath. It's a great watch if y'all haven't seen it.
Men who are very sexy love women, are comfortable with women, are completely at ease with women, and yet are not perceived as docile by women. Be the guy who might smile and flip her skirt up in an alleyway. Be the guy who asks the woman next to you with a sly grin whether you should take the gal across the room home. Be the guy who is out with his buddies enjoying a cigar ignoring the hot women sitting right next to him (because after all hot women are as normal as breathing). Be the guy who says to a woman with a warm smile "You're ALL crazy darling, every one of you." It is an unwavering acknowledgement of the masculine sexual nature. It is mental and builds sexual tension. It can be charming and debonair, it can be genteel even. It can be crass, it can be nerdy. But
it all comes from the unapologetic place of I AM MAN; I AM SEXUAL in many non-verbal cues, which a woman reads on a subconscious level all at once.
It is never insecure. Ever. It can vulnerable but never insecure.
Two men on SS that I am quite sure are like this in real life are
@Bible_Belt and
@bradd80. It's not that you are Adonis or perfect, in fact it's the imperfections and flaws that add the authenticity and the charm. Both BB and Bradd come across as a mess in their own way and for different reasons. They are players; they just need saving. (They don't really - but this is part of the appeal.) This is why women adore the rake. The rake is what he is, he simply cannot help himself and it is endlessly seductive for a woman to think she will be the one to reform the rake.
Usually the true rake cannot be reformed nor possessed and thus he is endlessly unobtainable and endlessly fascinating. He is also endlessly frustrating and is often quite alone at his core. While he cannot be possessed he can be understood if a woman has enough self assurance to know what she is dealing with (most do not.) Ladybird Johnson understood LBJ in this way. LBJ had lots of mistresses and dalliances. But Ladybird knew she was THE WIFE, the confidant, the one who "got" him. And she didn't worry about his floozies at the end of the day.
To go to the extreme of the true rake may not be in a man's best interest (depending on his goals).
But a sexy man is always a man with an attitude of abundance, he always has the ability to dial up options, and even if he is married his wife is well aware that other women would happily swoop in and take her place. So women want to please this sort of man, for he must be pleased since he is choosing to choose her and she best not become controlling or complacent.
Women can have this kind of sexual vibe too. Marilyn Monroe obviously had it. So did Mae West, Sophia Loren, and Raquel Welch; the best current example of it in Hollywood right now is Margot Robbie IMO. The overt sexuality of sexual confidence is inherently masculine, even if it is exhibited by a woman. Look up the Rake, the Siren, the Masculine Dandy (a woman) and the Feminine Dandy (a man).
So it is not just confidence. It is unapologetic sexual confidence. As a man you can develop your sexual confidence by building your confidence in other areas of your existence and by building your comfort level around women (get rejected; get a woman friend to wing for you; get used to saying what you think and not pandering or apologizing if some feminist makes a face; etc.) And for heaven's sake do not do it in a professional environment...but
sexual confidence is what comes off as sexiness. 
