The story
From giant + slalom. * (skiing): Being a slalom with larger turns. * (snowboarding): From its similarity of turn with skiing's giant slalom.
Etymology from Wiktionary, CC BY-SA 4.0.
giant slalom
Today it means: An Alpine skiing discipline in which the course is longer, steeper, and faster than normal slalom, and the gates are spaced at a greater distance. Full definition →