From Middle Englishtofal, equivalent to - + fall. Compare Dutchtoeval, GermanZufall, Danishtilfælde, Swedishtillfälle, Icelandictilfelli. More at to, fall.
(archaic, architechture) A shed or building annexed to the wall of a larger one, having its roof formed in a single slope with the top resting against the wall; lean-to.