A combination of possibly and probably. If possibly gives something a chance from 0% to 50% of happening, and probably gives something a 75% or greater chance of happening, then prossibly is the area between the two, the range of 50% to 75%.
Person 1: Hey, you want to go to the movies on Friday?
Person 2: Prossibly, depends on whether or not my boyfriend has to work.
Person 1: Dag, you can't let him control your life.
Person 2: But he loves me!
Person 1: Lame.