“Jockstraps and suspensions” (Ad for Will & Finck, Headquarters for Sporting and Athletic Goods). San Francisco Examiner, January 25, 1891, p5
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1919. “You can enjoy perfect comfort and freedom. Schnoter’s Suspensories and Jock Straps have gained their widespread popularity by quality, wear, fit and low price” (The Billboard [Cincinnati], Dec. 20; Peter Tamony).