I learned this one a long time ago when I was studying for the Kanji Kentei and came to 峠. It’s got an interesting little bit of history bundled up in it, which I like.

洞ヶ峠を決め込む
(Horagatouge wo kimekomu; “Staying put in Horagatouge”)

Definition:

Waiting to see how the wind is blowing before making a move. Checking to see which side in a conflict is winning and then throwing in with them. Double-dealing. Being a fair-weather ally.

This saying is tied to the Battle of Yamazaki, where Toyotomi Hideyoshi faced down Akechi Mitsuhide after the latter’s betrayal of Oda Nobunaga. The story goes that Tsutsui Junkei, a nominal ally of Akechi, kept his men at the eponymous pass while he observed which way the battle seemed to be going, then switched allegiance and threw his lot in with the Toyotomi side when it became clear that they were going to win.

Other sources remark that while the story is dramatic and memorable, it is also likely false: historians apparently believe that Tsutsui’s forces were some distance away and would not have made it to the battle in time to throw their lot in with either side. I suppose it’s possible that their absence, bitterly noted by an Akechi ally and later spun out of context, is the germ of the story and this saying.

Breakdown:

洞ヶ峠 is a proper noun, the name of a place called Hora-ga-touge. 洞 (hora) is a cave; 峠 (touge) is a mountain ridge or pass. The character ヶ is an odd beast. While it looks like the katakana phonetic character ケ (ke), it’s actually a sort of contraction symbol that can stand in for several other characters, most of which take more strokes to write. In this case it stands for (and is pronounced the same as) the particle が (ga), which in older grammar served the associative function filled in modern Japanese by の (no). The place name would therefore be literally rendered as “Cave Pass.” It’s marked as the direct object of the following verb by the particle を (wo).

The second major term in this kotowaza is the compound verb 決め込む (kimekomu). It comprises the conjunctive form of the verb 決める (kimeru), “to decide” and the sentence-final form of 込む (komu), “to be crowded,” “to go into,” “to do thoroughly.” The compound’s meaning is a bit more complicated, though: it can mean “to assume / to pretend that something is true” or “to (intentionally) persist in doing [something].”

Notes:

This saying can be evoked simply by the place name 洞ヶ峠; in some dialects the place name is apparently used to indicate a liar.

You can read more about ヶ here.

Example sentence: (Note that I changed the final verb to a negative form!)

「あいつもこの事件に巻き込まれてしまったから、俺らも、洞ヶ峠を決め込まずにあっちの見方になるしかないと思う」

(“Aitsu mo kono jiken ni makikomarete shimatta kara, orera mo, Horagatouge wo kimekomazu ni acchi no mikata ni naru shika nai to omou.”)

[“Since that guy got caught up this business, I guess we have to join that side too, instead of just hanging back and seeing how things turn out.”]