1560s, "unbecoming, in bad taste," from French indécent (14c.), from Latin indecentem (nominative indecens), from in- "not, opposite of, without" (see in-(1)) + decens (see decent). Sense of "offending against propriety" is from 1610s. Indecent assault (1861) originally covered sexual assaults other than rape or intended rape, but by 1934 it was being used as a euphemism for "rape." Related: Indecently