"saddle made for the occupant to ride on with both feet on the same side of the horse," used chiefly by women, late 15c., from side (adj.) + saddle (n.). As an adverb, in ride side-saddle, by 1885.
Entries linking to side-saddle
side adj.
late Old English, "long, broad, spacious; extending lengthwise," from side (n.). Compare Old Norse siðr "long, hanging down." From late 14c. as "being from or toward the side," hence also "subordinate." Also "apart from the main course" of anything, as in side-road (1854); side-trip (1911). In side-eye (by 1922) the notion is "directed sideways."
saddle n.
Middle English sadel, from Old English sadol "contrivance secured to the back of a horse, etc., as a 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.
Extended to various things resembling or functioning as a saddle. Figurative phrase in the saddle "in an active position of management" is attested from 1650s. Saddle-horse "horse for riding" is from 1660s. Saddle-stitch (n.) is from bookbinding (1887).