slin pins = insulin needles
These come as a fixed unit, needle permanently attached to the syringe. Usually very fine gauge, 27 and up. Also typically a short needle (you can find slightly longer ones, but they’ll still be no bigger than an inch usually). Used for insulin (duh), HGH, and other peptides which get injected subQ (subcutaneous - directly beneath the skin). I (personally) also use them for my TRT injections (usually in lat or delt, two very low BF areas so the needle will make it to the muscle) since the volume of each injection is so small.
Used that term last night, and thought I’d add it to your list.