late 14c., molesten, "to cause trouble, grief, or vexation, disturb, harass," from Old French molester "to torment, trouble, bother" (12c.) and directly from Latin molestare "to disturb, trouble, annoy," from molestus "troublesome, annoying, unmanageable," which is perhaps related to moles "mass" (see mole (n.3)) on notion of either "burden" or "barrier." Meaning "sexually assault" is attested by 1950. Related: Molested; molesting.
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Harper, D. (n.d.). Etymology of molester. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/molester