What does 丫 mean? 丫是什么意思？
It is a short word for "丫头养的" only used in area around Beijing city，"丫头" means slave girl or young girl servant. "养的" means feed by. 

This is bad language saying somebody's mother is young girl servant, some kind of "son of bitch". However this talking is generalized nowadays, "你丫..." is more compare to "You fucking ..."(你他妈的...)