Turns out that’s kind of rude with humans too, eh
(Sedo no uma mo aikuchi;
“Even a back-entrance horse has a friend”)
“Even an unruly horse can be tamed.” Even the most unfriendly, poorly-socialized person can become relatively cooperative if approached the right way. The image is of a horse so wild or malicious that it must be taken to the rear entrance of an estate (because taking it through the main entrance would get in people’s way or otherwise cause problems) – and how if handled correctly, even that horse will behave docilely.
This terse saying begins in the middle with the noun 馬 (uma), “horse.” The associative particle の (no) connects this to the compound noun 背戸 (sedo), literally “back door,” i.e. the rear entrance of an estate. The emphatic particle も (mo), “also,” “even,” in turn connects the resulting noun phrase to the noun 相口 (aikuchi), literally “mutual mouth,” i.e. a friend; someone willing to spend time chatting together. One may imagine an elided copula at the end.
This saying has apparently fallen into disuse and doesn’t appear at all in most of my usual sources, but is apparently the せ (se) entry of the Osaka iroha karuta set.
Aikuchi may also be written as 合口 or 合い口 without any change in meaning or pronunciation. There is also a homophone 匕首, but this refers to a short dagger-style weapon and would be an error if used in this saying.
(“Sedo no uma mo aikuchi dakara, biiru de mo ogottara nakayoku nareru ka to omotta no ni, ainiku ano shimattsura yarou wa geko dattaa.”)
[“They say that even a wild horse can be tamed, so I thought that I could get on his good side if I treated him to a beer… but of all the luck, it turns out that that scowling a-hole doesn’t drink!”]