Wi-Fi Signal Stronger

Alright, listen up, you big ol' dum-dum! This joke is making fun of how skinny someone is. You know how a Wi-Fi signal can be weak or strong, right? Well, this joke is saying the person is so incredibly thin that even something invisible and often weak, like a Wi-Fi signal, has more 'strength' or 'substance' than them. Get it? You probably thought Wi-Fi signals were made of actual muscles, didn't you, you big oaf?