a "COBA" (pronounced: côh-bá) occurs when an attractive person is associated with an ugly or "unattractive" person, and said ugly guy or girl gets checked out, just because he/she is physically with a good-looking or even "hot" human being.
A COBA is a very tricky thing, if a COBA is initiated and the ugly person (who is usually checked out last) believes it was for their own looks, it may give the ugly one a false sense of confidence, that may lead to terrible choices of fashion; in clothing, hat selections and even worse, terribly unjustified arrogance. But, when COBA is noticed for what it is, the unattractive person can instantly return to his/her cyle of quiet self loathing.
Whether it be for selfish reasons i.e: making themselves look better in comparison (see also; duff) or the more noble paths: need of wingman/wingwoman, actually likes ugly person as a person: the better looking person always has a reason to be with their homely counterpart. Someone who has done a COBA can read the bond between the two, then analyze it from there.
Juan: "No way man, she is SO not into fat dudes, plus, she friggin' eye-raped you first, that was a total COBA"
David: "a what?"
Juan:" a C.O.B.A!: check out by association, she checked you out cos' she wanted to, and me because she wanted to try to comprehend our friendship, and/or why you'd hang out with an ugly guy."