Insert or introduce into the course of something, interject, as in He always threw in a few jokes to lighten the atmosphere. [c. 1700]
Add something with no additional charge, as in The salesman said he'd throw in the carpet padding. [Second half of 1600s]
throw in with. Enter into association with, as in His friends warned him against throwing in with the notorious street gang. [Second half of 1800s] Also see cast one's lot and the subsequent idioms beginning with throw in.