Robert to Bob, Richard to Dick is due to the R/D thing. The first name of the bassist for the band Rush is Gary...when his mamma called him, she sounded like she was saying "Geddy", hence Geddy Lee.

But, I still can't make the walk from John to Jack...anyone? Bueller?

And yes, I know the OP was looking for the origin.

The earliest reference I saw among the responses was to an Eddie Murphy standup routine, which goes back to the early 80s.

My comment was just adding some colour to the use discussion since I hadn't seen anyone mention "I'm just throwing it out there", which is also a not-uncommon phrase.

I use it synonymously with "I'm just throwing it out there." In other words, after saying something that might be considered offensive or controversial, yet also reasonable/logical/understandable, tacking on "I'm just sayin'/I'm just throwing it out there" distances the statement from me personally, so that the topic itself can more easily be discussed for whatever merit there may be in the suggestion.

This intends to a) allow someone to contend the argument without fear of hurting my feelings, since I've already offered the distance; and b) allows me to have even made the statement without fear of reprisal because, hey, it's not how I might actually see it, it's just an idea for your consideration.

Reminds me of Chris Rock summarizing OJ's position with "now, I'm not saying he did it...BUT I UNDERSTAND."

As an example, the continuing tensions between N. Korea and S. Korea prompts me to compare N. Korean Twitter huffing and puffing to that of a schoolyard bully, and you didn't get a schoolyard bully to pipe down by continuing to hand over your lunch money....I'm just sayin'.