"attentive, devoted, constant in application," 1530s, from Latin assiduus "attending; continually present, incessant; busy; constant," from assidere/adsidere "to sit down to, sit by" (thus "be constantly occupied" at one's work); from ad "to" (see ad-) + sedere "to sit," from PIE root *sed- (1) "to sit." The word acquired a taint of "servile" in 18c. Related: Assiduously; assiduousness.
