その手は桑名の焼き蛤
(Sono te wa Kuwana no yaki-hamaguri; “That trick won’t work!”)

Definition:

This one is a pun! It has a serious meaning, saying that someone’s strategy or trickery (often verbal) isn’t going to succeed (usually against the speaker). But where one might normally use the verb 食う (kuu, here meaning “to be negatively affected by something”) in its negative form kuwanai, the verb is switched out for the place-name Kuwana. Kuwana was apparently known for its roasted clams, so a mention of those is tacked on to the end for an overdone, playful feel.

Breakdown:

We begin with その (sono), generally translatable as “that” in that it often indicates something physically or psychologically closer to the listener than to the speaker. As a determiner, it identifies the following noun: 手 (te), “hand,” which in turn is marked as the focus of discussion by the topic-marker particle は (wa).

This is where you’d get your negative verb, 食わない (kuwanai), as noted above, but instead you get 桑名 (Kuwana), which literally means “mulberry name.” The associative particle の (no) connects this to the noun 蛤 (hamaguri), the “common orient clam.” This noun is modified by the verb 焼く (yaku), “to bake / to roast,” in prenominal form.

Notes:

Te is a normal metaphor in Japanese for a plan, strategy, trick, or even a “move” in a board game.

This bit of humor was apparently in relatively common usage by the Edo period.

Example sentence:

「相手の狙いに見当がついて、その手は桑名の焼き蛤だぞと思ったけど、おかげで油断してしまった。それこそ相手の仕掛けてた罠なのかな」

(“Aite no nerai ni kentou ga tsuite, sono te wa Kuwana no yakihamaguri da zo to omotta kedo, okage de yudan shite shimatta. Sore koso aite no shikaketeta wana na no ka na.”)

[“I had an idea of what my opponent was aiming at and thought to myself Oh no, you’re not pulling one over on me that easily! But thanks to that, I let my guard down. Was it a trap that they were laying for me all along? I wonder….”]