early 15c., diminishen, "to lessen, make or seem to make smaller," from merger of two obsolete verbs, diminue and minish.
Diminue (late 14c.) is from Old French diminuer "make small," from Latin diminuere "break into small pieces," variant of deminuere "lessen, diminish," from de- "completely" (see de-) + minuere "make small" (from PIE root *mei- (2) "small"). Minish (mid-14c.) is from Old French menuisier, from Latin minuere.
Meaning "to lower in power, importance, or estimation" is from mid-15c. Intransitive sense of "to grow less, become or appear smaller" is from 1510s. Related: Diminishes; diminishing.