臍で茶を沸かす
(Heso de cha wo wakasu; “Boil tea in the belly button”)

Definition:

Something is so funny – or by extension, so hilariously stupid – that you can’t stand it. The image is of laughter so paroxysmal and intense that the belly twists and jumps like water being boiled for tea. It’s best not to use this phrase for innocent laughter, though – it’s commonly used to mock someone who has unintentionally made you laugh with their stupidity.

Breakdown:

A very simple subject-object-verb sentence. First comes the noun 臍 (heso), “belly button,” marked as the location of the action by the particle で (de). Next comes the noun 茶 (cha), “tea,” marked as the direct object of the verb by the particle を (wo). And the verb is 沸く (waku), “to become hot,” “to boil,” in imperfective form, with the causative suffix す (su) in sentence-final form.

Notes:

The location-marker particle で may be replaced with subject-marker particle が (ga), or the whole phrase shortened to 臍茶 (hesocha), but replacing 茶 with the more pedestrian 湯 (yu), “hot water,” is considered an error.

Example sentence:

「最近上映になったコメディーは見づらいながらも面白いね。主人公は臍で茶を沸かすレベルのボケで上手く演じられてるんだ」

(“Saikin jouei ni natta komedii wa midzurai nagara mo omoshiroi ne. Shujinkou wa heso de cha wo wakasu reberu no boke de umaku enjirareterun da.”)

[“That comedy that recently started showing is hard to watch, but funny. The main character is well-acted as a side-splittingly hilarious idiot.”]