half joking; half in jest
Used when something is said or done partly as a joke, implying the speaker is not entirely serious. Often appears in adverbial phrases like 冗談半分に.
He said he would quit, half joking.
The suggestion was half in jest, but surprisingly it was adopted.
冗談 means 'joke' or 'jest', while 冗談半分 specifically means 'half joking' or 'not entirely serious'.
本気半分 means 'half serious', the opposite nuance of 冗談半分.
Compound of 冗談 (joke) and 半分 (half). The exact historical derivation is uncertain, but it follows a common pattern for yojijukugo expressing mixed states.