i'll be a monkey's uncle - You say this when you're so amazed you wish you had a monkey

You say this when you're so amazed you wish you had a monkey for a pet instead of a bad grade.

'You finished the whole homework? I'll be a monkey's uncle!'

'He just got a scholarship? I'll be a monkey's uncle!'

'You just texted me during the test? I'll be a monkey's uncle!'