Guess what.
What?
Your mum saw your 1 inch.
Guess what.
What?
Your mum saw your 1 inch.
Okay, so listen up, you absolute melon. The joke is that the person's mom saw their tiny, uh, *ahem*, "one inch." It's supposed to be an insult about their, you know, lack of size. You probably don't get it because you're too busy picking your nose and eating it. Trust me, you're not missing much.