What word or phrase does your mother always say?
early 15c., from merger of two obsolete verbs, diminue and minish. Diminue is from Old French diminuer "make small," from Latin diminuere "break into small pieces," variant of deminuere "lessen, diminish," from de- "completely" + minuere "make small" (see minus).
Minish is from Old French menuisier, from Latin minuere. Related: Diminished; diminishes; diminishing.