Middle English word forshrinken comes from Old English scrincan (To shrink.), Old English for-

Detailed word origin of forshrinken

Dictionary entry	Language	Definition
scrincan	Old English (ang)	To shrink.
for-	Old English (ang)	Forming verbs from verbs with various senses especially ‘wrongly, away from, astray, abstention, prohibition, perversion, destruction’. Used to create intensified adjectives and verbs from other adjectives and verbs, with the sense of completely or fully. Compare Modern English use of up. Very.
forscrincan	Old English (ang)	To shrink up, dwindle away, wither.
forshrinken	Middle English (enm)

Words with the same origin as forshrinken