noun
joking manner; joking tone
Refers to speaking in a joking way, often used in phrases like 冗談口をきく (to speak jokingly) or 冗談口で言う (to say something in a joking tone).
彼はいつも冗談口で話す。
He always speaks in a joking manner.
冗談口をきいて、場を和ませた。
He lightened the mood by speaking jokingly.
冗談 is a general noun for a joke or jest, while 冗談口 specifically refers to a joking manner of speaking or a joking tone.
Compound of 冗談 (joke) and 口 (mouth, manner of speaking). The exact historical derivation is uncertain, but the word is transparently formed from these two elements.