In marijuana culture, a fink is someone who says they will match you and comes up with an excuse, often unrealistic to avoid pithing in marijuana. Because they promise to pitch in weed and do not, they are more hated than sponges. A fink is recognizable due to their lack of moral ethics in general, paranoid nature and endless preoccupation with how they are viewed by others.
Guy 1 "Man, I can't believe Guy 3 tried to pretend the wind blew his weed away and didn't pitch in for that bowl,"
Guy 2 "Thats nothing. Yesterday he not only didn't pitch in, he jacked my lighter after the session and tried to pretend he didn't. I nearly punched him when I found out."
Guy 1 "Man he is sucha fink."
Guy 2 "Agreed"
Guy 3 *enters on scene*
"Hey guys want to smoke some weed? i got some prime buds, but they are at home so we can smoke then go get them."