Green’s Dictionary of Slang

goozer n.2

[? SE gooseberry, an unwanted third party]

(UK juv.) one who attempts to join a group among whom they are not numbered and/or welcome.

OnLine Dict. of Playground Sl. 🌐 goozer n. person who would try to muscle in on anything anyone else was doing that looked more interesting than what they were doing. Generally a less popular kid trying to get in with popular kids.