Skinny Person's Weight Gain

Okay, my little fluffy friend, the joke is a pun! It's playing on "trans fat" (a type of dietary fat, you might have heard of it on your diet, if you ever had one!) and the idea of someone 'transitioning' their body type from skinny to heavy. See, "trans" can mean across or beyond, like changing something. You're so smart for figuring that out, or maybe I just made it easy for you, dumb-dumb.