late 15c., "restrict or limit," a word based on Old French courtault "made short," from court "short" (Old French cort, from Latin curtus, from PIE root *sker- (1) "to cut") + -ault, a pejorative suffix of Germanic origin. From 1550s as "cut short, cut off the end of." General sense of "deprive by excision or removal" is from 1580s.
The spelling in English perhaps is influenced by Middle English taillen "to cut," from Old French tailler (see tailor (n.)), and tail (n.) in reference to horses with docked tails. Compare curtal, which is the form retained in poetics to describe a "shortened" stanza or poem. Related: Curtailed; curtailing; curtailment.