A "bebo" is when you take your index finger and poke someone, multiple times if possible, in the buttocks, followed by saying "Bebo!" (I wouldn't recommend doing it to anyone besides sibilings)
Bebo is a word typically used to describe a hot guy, in Argentina. Any guy can be a bebo, but you only you and your friends know who really is one, and who is just a victim of an inside joke.