My nine year old nephew uses the term amn't a lot. When he first used it we were so impressed that his clever mind had come up with a contraction for 'am not'. Oh, we thought he was so clever to invent it, even if it weren't grammatically valid. For some reason I finally looked it up, and....WHAT?!?! It is an actual and valid contraction, though the pronunciation is different than how Jacob says it.
Main Entry: amn't
Pronunciation: \ˈänt, ˈant, ˈa-mənt\
Date: 1618
chiefly Scottish & Irish : am not
So, my new goal is to use the contraction when I have occasion. Wikipedia has a little to say on the subject. It is not only a contraction for "am not" but also "am I not".
Amn't I clever?