How did 'probably' become 'prolly'?

This reminds me of a funny story. I was asking my brother Josh if he was going to be able to make it to a party I was having.

Josh answered, "Prolly not."

I said, "Prolly?! Really?"

"Oh, sorry, 'probly' not.!"

I laughed out loud and said, "Don't you mean 'probably'?"

He just laughed and said, "Yah, that, proBABly."

I think the story illustrates how it happened. "Probably" with two Bs is a bit of a lip buster, if not a tongue twister. So, it became "probly." Then, somehow, "probly" digressed into "prolly." At the same time it developed that weird vowel at the beginning that sounds less like the O in "probably" and more like the dentist telling you to say, "Ahh," because you'd think it'd rhyme with "trolley," but it doesn't.

I live in Michigan, the state deficit of Bs that are owed to probably is staggering, prolly in the trillions.

I know when I see this I tend to wonder if I am spelling "Probably" wrong.. I am not in the loop of the latest slang.