It's a unit of mass. Pound is technically a unit of force, though for everyday use we treat it as if it was a unit of mass. Sometimes we used lbf and lbm to distinguish the two, but IIRC slug is the fundamental unit of mass in the Imperial system. Honestly, haven't had to deal with it in 30 years so it's all a bit fuzzy.

Have a pity for those of us in Canada who had to learn in the easy way (metric) and also the American way. Eventually I just converted everything to metric at the start, did the calculations, then converted back at the end. So much easier.

But I always liked the unit slug. Though from the name, sounds like it should be a unit of force, not mass, lol.