Colloquial definition of "douchebag"
Solution 1:
For the usage you're thinking of, I would suggest a definition like "someone who acts self-consciously cool, hip, or sexually desirable in a highly noticeable and aggressive way." Self-consciousness is an important part of the definition: he knows he's doing it and is trying a little too hard. In my experience there's also a definite social-class element to it as well (what one might call an "overgrown frat boy vibe").
Like many slang expressions, though, douchebag is probably better defined by example than through any formal attempt at definition.
Solution 2:
"Douchebag" as an epithet was originally coined in the Eastern Seaboard area as a general pejorative term for a woman. The male-oriented counterpart term in that sense is "scumbag", a reference to a used condom and nowadays a fairly mild epithet.
The term has evolved over the years and crossed genders, much like the word "bitch" used to be used to refer to women only (replacing the archaic but similar term "shrew") but is now applied to men. The former gender-specific nature of the word is likely exactly why such words cross genders; not only is the word still derogatory, but doubly so as the speaker implies the subject is unmanly.
The term in its most popular use is a furtherance of general metaphors equating a person with something disgusting, such as someone being called "pond scum", a "turd" or an "asshole". The inference is that the subject is socially repulsive in general. "Douchebag" specifically will generally conjure up the image of a totally self-centered, probably well-to-do, but socially and sexually immature person; refer to phenry's "overgrown frat boy".
Solution 3:
In its original context, the term "douchebag" refers to a form of "birth control" or actually a preventive measure taken AFTER the (f)act (that is, after the sperm has been spilled).
In a figurative sense, a douchebag refers to an impulsive person who lacks self-control, and who drinks, parties, or"dates" in excess, and therefore has to be "cleaned up" afterward.
Solution 4:
I quickly found two urbandictionary definitions that I think are good starters:
-a self-promoting asshole who contributes nothing to the world. Tool. Loser
-Guy who gets the girl but doesn't deserve her
Douchebag can often just mean "a jerk", a general insult. I think it does have connotations of the type of guy that hits on a lot of girls, and while he projects sexual charisma, is very conscious of his image. A douchebag is certainly self-promoting and selfish. Almost certainly has little respect for women.
I think there are a lot of characteristics you could list because "douchebag" is a type of person.
Solution 5:
A word to describe an individual who has shown themself to be very brainless in one way or another, thus comparing them to the cleansing product for vaginas.
Source: Urban Dictionary (10649 up, 1233 down).