Merriam-Webster's Collegiate® Dictionary, 11th Edition
SKI
I. noun (plural skis; also ski) see: shed
Date: 1755
1. one of a pair of narrow strips of wood, metal, or plastic curving upward in front that are used especially for gliding over snow, water ski , a piece of material that resembles a ski and is used as a runner on a vehicle,
II. verb (skied; skiing)
Date: circa 1890
intransitive verb to glide on skis in travel or as a sport, transitive verb to travel or pass over on skis, skiable adjective skier noun