The story
From lap (“upper legs of a seated person”) + sash (“length of cloth worn over the shoulder”) + seatbelt (“restraining belt”).
Etymology from Wiktionary, CC BY-SA 4.0.
lap sash seatbelt
Today it means: A type of seatbelt which goes across the pelvis and then up and diagonally across the body; the conventional type found in most cars today. Full definition →