half-joking; half in jest
Used when someone says something that is partly serious and partly a joke. Often appears in phrases like 半分冗談で言う or 半分冗談半分本気.
He said that half-jokingly.
That was a proposal that was half in jest and half serious.
Compound of 半分 (half) and 冗談 (joke). The exact origin is straightforward; it follows a common pattern of combining a quantity with a noun to describe a mixed state.