Probably from Old French 'estaler' (to stand still) — stale things have been standing too long.
No longer fresh; old and lacking interest.
Probably from Old French 'estaler' meaning 'to come to a standstill,' related to 'estal' (a standing place, stall). Stale food has been standing too long — it has stopped being fresh. Key roots: estal (Old French: "standing place, stall").