Cookie Mouse 

Someone who asks for more than what was already given to them

In the movie School of Rock, Jack Black’s character, Dewey Finn, was a cookie mouse because he mooched off his roommate, Ned for free rent

Cookie Mouse by MinDATucker March 22, 2022

if you give a mouse a cookie 

This is a term used to refer to people who push themselves into your house as soon as you let them put their put in the door. Metaphorically speaking. If you give someone an edge of hope or anything, they will try to take advantage of that and get more.

Sally: Ugh...so my prom date wants to take me out on a date. Like more than friends.

Brenda: Well, if you give a mouse a cookie...

if you give a mouse a cookie by Wise Gal March 27, 2012