(historical) A cake used in a fundraising game, in which individuals pay money to guess the weight of a cake (or, in other versions, to guess the identity of an object baked into the cake), with the cake going to the winner.
1892, “Whispers”, in Locomotive Engineers' Monthly Journal, page 334:
The principal feature of the evening was a grab bag and a guess cake, from which the ladies realized a handsome sum for the treasury.