i'll be a monkey's uncle - This phrase comes out when you're so surprised you'd rather

This phrase comes out when you're so surprised you'd rather be stuck with a monkey than your worst enemy.

'You just got a 100 on the quiz? I'll be a monkey's uncle!'

'He just said no to the pizza? I'll be a monkey's uncle!'

'You just told the teacher about my doodles? I'll be a monkey's uncle!'