жартома́

I said it by way of a joke.

He said it as a joke.

Tom said jokingly that he was not very rich.

I said it in jest.

I said that as a joke.

Sami did that as a joke.

White pawn to f3, black pawn to e6, white pawn to g4, black queen to h4 - checkmate! This is jokingly referred to in chess circles as "fool's mate".