Green’s Dictionary of Slang

(orig. Aus.) silly, foolish.

1957		Kerouac On The Road (1972) 237: What’s your road, man? – holyboy road, madman road, rainbow road, guppy road, any road.
1976		J.W. Dean III Blind Ambition 139: I [...] chastised myself for having seemed naïve and guppylike at times, but I knew I was learning.
2001		OnLine Dict. of Playground Sl. 🌐 guppy n. A generic term, ‘guppy’ was a bit of wild card insult that could mean almost anything derogatory -- thick, stupid, unpopular, unlikable, ugly etc.