My wheelchair-bound friend was getting bullied, so I told him to stand up for himself.
Stand Up For Yourself
Explanation
Explain Bear
Alright, listen up, buttercup. The joke is that the dude is telling his friend in a WHEELCHAIR to STAND UP. Do you get it, bonehead? It's funny because the friend CAN'T stand up. Now go back to eating your boogers, you Neanderthal.