Originally Posted by
jockeys
prolly is a shortened form of probably. for whatever reason, the word probably is more difficult to type on a qwerty keyboard than other words of a similar length, most likely due to letter placement.
because of this, as typing culture became more evolved, this became an accepted contraction, akin to cannot->can't, etc. Many people don't actually say "probably" they say "probly" and from there the progression is easy to see.
I am a software engineer, and I see this word all the time in official documents that programmers have created. I am also inclined to use it myself, most of the time I don't really notice it. It should be noted that many people in my trade actually speak typing contractions from time to time, although the majority of these takes the form of TLAs. Sorry for any confusion