飼い犬に手を噛まれる
(Kaiinu ni te wo kamareru;
“Bitten on the hand by your pet dog”)

Definition:

To be betrayed or harmed in some way by someone you had deeply cared for, or taken care of on a regular basis. (In other words, it doesn’t apply to a stranger whom you once did a favor for, etc.)

Breakdown:

We begin with the verb 飼う (kau), “to keep / raise / feed (an animal),” in conjunctive form, which allows it to function as a noun. This noun compounds with, and modifies, the noun 犬 (inu), “dog.” Direction particle に (ni), “from,” marks the dog-one-raised as the source of the verb 噛む (kamu), “to bite.” This verb appears in imperfective form and takes the passive formation れる (reru). The direct object of the “being bitten” is marked by particle を (wo); this object is the noun 手 (te), “hand.”

Notes:

This is interesting, because in English we have a parallel saying, “to bite the hand that feeds you.” The content is almost exactly the same (although the English lacks the canine specificity and could apply to a one-off favor being repaid with harm), but from the opposite point of view.

Example sentence:

「例の上院議員は誰よりも先にあの革命的な政治家を支援した一方で、誰よりも罵詈雑言を浴びされて、誰よりも先に棄てられた。一言も口は利かないが、脳裏で飼い犬に手を噛まれたと考えているんじゃないかな」

(“Rei no jouin giin wa dare yori mo saki ni ano kakumeiteki na seijika wo shien shita ippou de, dare yori mo bari zougon wo abisarete, dare yori mo saki ni suterareta. Hitokoto mo kuchi wa kikanai ga, nouri de kaiinu ni te wo kamareta to kangaeteiru n ja nai ka na.”)

[“Although this senator gave his support to that revolutionary politician before anyone else, he was also showered with abuse more than any other, and thrown away before any other. He doesn’t say anything about it, but in the back of his mind he must feel like he was bitten by the dog he’d raised.”]

Nichijou