What up my homies is a casual term used by "hip and swag" white boys that are trying to act cool. "What up" is a term that defines a greeting. "my" is something that is yours. "homies" Another word for "the boys".
(Enters into COD lobby with friends)
big_dingdong69: yo
1080noscope: was up bro
FaZe knArlY: yo yo my guy
"You": what up my homies