The story
Probably sag + wagon; sometimes also said to stand for "support and gear" or "support and grub" (i.e. food).
Etymology from Wiktionary, CC BY-SA 4.0.
sag wagon
Today it means: A truck, van or other support vehicle used during cycling events, designed to hold food and other equipment for participants, and to pick up and carry any cyclists (and their bicycles) who can no longer ride. Full definition →