Green’s Dictionary of Slang

(US campus/teen) anything or anyone seen as amusing; often intensified as stitch and a half.

[1850		Man of Pleasure’s Illus. Pocket-book n.p.: This caper is about the neatest stitch we have tumbled to. There is something new in the bilk—'tis not the musty dodge].
1989		(con. 1960) P. Theroux My Secret Hist. (1990) 125: ‘You’re a stitch!’ She shook her head and was laughing.