A corruption of crustade 'a pie with a crust' — the name migrated from the crust to the egg filling inside it.
A dessert or sauce made with milk, eggs, and sugar, thickened by gentle heating.
Alteration of crustade 'a pie with a crust,' from Anglo-French crustade, from Old French crouste 'crust,' from Latin crusta 'rind, shell, crust.' The original custard was a pie — the name transferred from the crust to the filling, and then the crust was forgotten entirely. Key roots: *krus- (Proto-Indo-European: "to form a crust, freeze").