early 15c., decresen (intransitive) "become less, be diminished gradually," from Anglo-French decreiss-, present-participle stem of decreistre, Old French descroistre (12c., Modern French décroître), from Latin decrescere "to grow less, diminish," from de "away from" (see de-) + crescere "to grow" (from PIE root *ker- (2) "to grow"). Transitive sense of "make less, lessen" is from late 15c. Related: Decreased; decreasing.
late 14c., "detriment, harm;" early 15c. as "a becoming less or smaller," from Anglo-French decres, from the verb (see decrease (v.)).