The story
Coined by Carlin Vieri in his time at the MIT AI Lab (1993–1998) after viewing a segment at the end of "The Boy Who Cried Rat!" (1991), the sixth episode of the first season of The Ren and Stimpy Show. The segment featured "Yak Shaving Day", a Christmas-like holiday where participants watch for the shaven yak to float by in his enchanted canoe.
Etymology from Wiktionary, CC BY-SA 4.0.