Musick... Can one just say “a Jimmy”? Is it at all well known?

It has come to mean exactly that... "a Jimmy".

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Here's a quick history.

My best friend's name, who I've known his whole life, is Jimmy. His dad was James, and now his son, James the 4th, has taken over the 'Jimmy' designation.

One of my other close friend's name is James, but he likes to be called Mitch (his middle name), and here's why. He was the youngest brother of three for quite some time as his parents created two sisters afterward, and then finally another brother. The family next door also had a youngest brother named James about the same age. They played together as the grew up together. They were known as 'the Jimmy's'. The joke/saying slowly caught on that anyone's younger brother was a "Jimmy", especially when you'd see someone on the street that kinda looked similar to someone you knew, and he was automatically his younger brother 'Jimmy'. Mitch needed a little *more identity than that when he started high school.

An ex co-worker, surprisingly enough, had a similar reference creep into his language, from what we can tell, out of entirely different circumstances, but in his usage "Jimmy" meant "a regular guy".

And finally, one of my recent "partner's in crime"(so to speak), his name is William James. He insisted on being called William and not Bill, so, quite naturally of course, he became "Jimmy". He calls me "James" (to remind me of his point).

At the place I work, I call people and am called "Jimmy" a number of times a week. It has become a term of comradery. When I'm asked why or what does it mean, I answer you're "one of the guy's" and "part of the team" and that they are "my buddy".

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insel - "Well known" is relative, I guess.