A small penis, a penis of small size. The owner of a 'dappa' is usually quite popular and good-looking, so most guys who know him are privately happy with the knowledge that he has a tiny 'dappa'
"Oh yeah, did you hear about Gertrude? She took home the captain of the footy team, turns out he has a dappa.'
by DadyC00l1 January 15, 2012