Old English sadol "seat for a rider," from Proto-Germanic *sathulaz (source also of Old Norse söðull, Old Frisian sadel, Dutch zadel, zaal, German Sattel "saddle"), from PIE root *sed- (1) "to sit" + Germanic suffix *-þra, used to form neutral names of tools. Figurative phrase in the saddle "in an active position of management" is attested from 1650s. Saddle stitch (n.) was originally in bookbinding (1887).
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.