When something is goodn't, then it means good but it isn't. Couldn't means could not, because the "n't" means not. So adding "n't" to good makes something good, but it actually isn't good. Its another way of saying bad, or garbage.
MTF: Wow, that song you made is so goodn't! That song is better then 90% of my music! I either make mid music or trash music!
MTF: JK, your song is goodn't.
SFA: Bruh.
Bob: Those idiots at Taco Bell messed up my damn order again! I should go over there and slap some goddamn sense into those stupid shits!
Dave: Sheesh. Good thing you don't keep guns in the house.
A contraction of the words goober and not. Goobn’t describes a foolish person (or critter) that is mischievous, but is annoying or bothersome to deal with. The term is typically used endearingly.
“You goobn’t! I can’t believe you would walk in front of a golem like that!”
“What did you just call me?”