More mildly, Stevia and sharpened spoons?

口に蜜あり腹に剣あり
(Kuchi ni mitsu ari hara ni ken ari;
“Honey in the mouth and a sword in the gut”)

Definition:

Saying things that sound good, but secretly harboring malice or enmity. Honeyed words and ill intent. Using pleasing words to make someone happy and create a sense of closeness while secretly hating them or ultimately meaning to do them harm.

Breakdown:

What we have here is a pair of parallel verb phrases. The first begins with the noun 口 (kuchi), “mouth.” This is followed by the noun 蜜 (mitsu), which denotes sweet fluids such as honey, nectar, molasses, etc. The 蜜 is followed by the verb あり (ari), “to be,” and preceded by the particle に (ni), which marks the mouth as the location in which the nectar bes, so to speak.

The second verb phrase follows the same pattern, with verb あり denoting the existence of noun 剣 (ken), “sword,” and the particle に locating its existence at noun 腹 (hara), “stomach.”

The fun part about the verbs here is that the form they appear in can be either conjunctive or sentence-final. The first one is clearly the former, but the second could be either, depending on whether the overall saying is used on its own or as part of a longer sentence.

Notes:

This comes to us from the Zizhi Tongjian (資治通鑑, in Japanese Shiji tsugan), an 11th-century CE Chinese historiography by famed scholar Sima Guang (司馬光, in Japanese Shiba Kou). Supposedly the phrase was coined to describe Li Linfu (李林甫, Japanese Ri Rinpo), a minister who flattered the emperor while undermining anyone he saw as a rival, to the overall detriment Tang regime that he was supposed to be serving.

Example sentence:

「お世辞に昇進で報いる社長は、直に口に蜜あり、腹に剣ある連中に囲まれ、自分の失脚を招くだろうさ」

(“Oseji ni shoushin de mukuiru shachou wa, jiki ni kuchi ni mitsu ari, hara ni ken aru renchuu ni kakomare, jibun no shikkyaku wo manuku darou sa.”)

[“The company president who rewards flattery with promotions is soon surrounded by those with honey on their tongues and daggers in their hearts, and so invites his or her own downfall.”]