Well...my last boss before I retired from the Marine Corps was an aviator (attack pilot) and everyone who worked for him either already had a call sign (because they were aviators too) or was assigned one based on some "quaint" foible or telling incident. I'm a fairly big guy, broad shouldered, wear glasses, and I can occasionally be kind of stubborn and short-tempered when provoked, so "Rhino" was sort of a natural fit, you might say. (In good fun afterward, the colonel sometimes used to kid me by asking if I'd felt the urge to charge any jeeps lately...)
I was a Captain at the time, so later, when I needed a screen name...
I have since had conversations with retired aviators about how these call signs get assigned (usually toward the end of flight school or at your first squadron). You don't ever get to pick your own. And if you try to change it, brother, standby!
Anyway, all I can say is that the gods of call signs were smiling down on me that day because it could have been far, far worse.
What happens if you join a new squadron where someone already has your call sign?
Incidentally, my Dad's was "Pretty Lights", earned on a P-51 sortie over Norway in 1944. He noticed some twinkling lights at the rim of the fjord they were flying up, and blurted out "What are those pretty lights up there?" over the radio. The squadron leader immediately veered into a turn, knowing it was flak aimed straight at them. I don't know if his sharp eyes made up for breaking radio silence, but that became his name in the squadron (No. 19, RAF).
-ccm