It;s idiomatic to English and does not really mean anything in German,
but if you wanted to translate it, you would have to say
Steck eine Gabel (in mich) rein. Ich glaube, ich bin gar.
(like: well done - as in cooking)
But as I said, it does not translate well into German.
You should say:
Mir reicht's! Oder: Ich habe genug!