A shortening of 'man bosoms'. Another form of moobs or moobies.
A: Did you see Ferg at swimming?
B: Biggest mosoms I've seen in a long time.
A: They're bigger than your sister's!
B: (Punches A)
Giving a handjob or handy.
It should be reasonably clear where the idea comes from...
A: So did you go with that hot chick I saw you with the other night?
B: She gave me some pretty good gear, I must say.
A: Nice.
She was giving gear to two guys at once, the slut.