Prossibly very actually means halfway between probably and possibly. I suspect that when most people see me use this term, they just assume it was a brain fart on my part, and proceed to read into it whatever amount of certainty and commitment they like.
But when I say prossibly, I really do mean halfway between probably and possibly. Sometimes probably expresses more certainty than I actually feel, while possibly seems to express less commitment than I'm actually giving.
By the way, by commitment, I mean how much I want to be there, and by certainty, I mean how likely I am to be there.
In my mind, probably, prossibly, and possibly express two axes: Level of commitment and level of certainty. For me, probably is strong certainty and commitment, possibly is weak certainty and commitment, and prossibly is used for either strong commitment and weak certainty. I guess for completeness I should have a fourth term for weak commitment and strong certainty, but in practice I don't want to admit to weak commitment, so I just use either probably or possibly to express that, depending on my level of certainty.