This is a term used by cave creek 'bros' or 'breas' It is used loosely but usually is replacing the world yes or probably. Can be shortened to just 'prolly' but so not as hardcore. If you say this you should drink alot, wear bandanas, have SRH snickered up cars, and either be or have bro hoes.
Bro 1: Are you going to Jenns sick party?
Bro2: Prolly. Prolly a fish. You?