early 13c., "to free from faults, rectify," from Old French amender "correct, set right, make better, improve" (12c.), from Latin emendare "to correct, free from fault," from ex "out" (see ex-) + menda "fault, blemish," from PIE root *mend- "physical defect, fault" (source also of Sanskrit minda "physical blemish," Old Irish mennar "stain, blemish," Welsh mann "sign, mark").
The spelling with a- is unusual but early and also is found in Provençal and Italian. In English it has been supplanted in senses of "repair; cure" by its shortened offspring mend (v.). Meaning "to add to legislation" (ostensibly to correct or improve it) is recorded from 1777. Related: Amended; amending.