Обсос = лох = loser Скорее всего из того же разряда, что и сосунок, типа "молоко на губах еще не обсохло". В моем детстве было достаточно часто употребимо данное слово на улицах, где я рос. Most likely you will never hear that in sphere of well-educated and civilized people. But if you will heard that about yourself just hit and run away as fast as you can)))) This is a speech of punks and sub-criminals.

It's a local, perhaps outdated slang, denoting in that case a not modern and luxurious dressed person. I've never heard it besides this film.

In Russian slang used this word to refer to ugly, not good think or person. If you using word in other context you mean different, function like a verb or substantive. Examples. сосать обсосать tjhanks

бедно одет плохо одет.

"Брат" это культовый фильм :)