number 4.
bird (bɜːd)
— n
1. any warm-blooded egg-laying vertebrate of the class Aves , characterized by a body covering of feathers and forelimbs modified as wings. Birds vary in size between the ostrich and the humming birdRelated: avian , ornithic
2. informal a person (usually preceded by a qualifying adjective, as in the phrases rare bird, odd bird, clever bird )
3. slang chiefly ( Brit ) a girl or young woman, esp one's girlfriend
4. slang prison or a term in prison (esp in the phrase do bird ; shortened from birdlime , rhyming slang for time )