We’re talking the early 80’s, so I’ve been dealing with this nickname for almost 30 years now.
I was over at my older brother’s house and he was on the phone to our maternal grandmother, Ida. They had been talking for a while when she asked him how his brother was doing. Having two brothers, he asked which one she meant.
Not that she was senile or anything, but according to my brother her response was “uh, you know…, Slim.” You know, kinda like how you call that big burly guy “Tiny”.
Anyway, my older brother found that incredibly hilarious, and the siblings have been calling me that ever since.