Old English sadol "seat for a rider," from Proto-Germanic *sathulaz (cf. Old Norse söðull, Old Frisian sadel, Dutch zadel, zaal, German Sattel), from PIE *sed- "to sit" (cf. Latin sedere "to sit," Old Church Slavonic sedlo "saddle;" see sedentary). Figurative phrase in the saddle "in an active position of management" is attested from 1650s. Saddleback is from 1540s.
Old English sadolian "to put a riding saddle on;" see saddle (n.). The meaning "to load with a burden" is first recorded 1690s. Related: Saddled; saddling.