Greedy guts
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used in casual conversation among friends or family, not in formal settings. 
My sister ate all the cookies, what a greedy guts!
Pro Tip 2/3Mild Teasing
Can be used playfully to tease someone who has a big appetite without offending them. 
You ate the whole pizza? You're a right greedy guts!
Pro Tip 3/3British Slang
More common in British English, might not be well understood by speakers of other English dialects. 
At the party, John was being a greedy guts, gobbling up all the snacks.