(Doku ni mo kusuri ni mo naranai; “Neither poison nor medicine”)
Something – or someone – that doesn’t matter. Something neither harmful nor beneficial; it doesn’t matter whether it’s there or not. Something that’s just kind of whatever – but note that when used in reference to a person, the tone is faintly nasty.
This phrase is built around three major components: the noun 毒 (doku), “poison,” the noun 薬 (kusuri), “medicine,” and the verb なる (naru), “to become” or by extension “to be,” appearing in sentence-final negative form. Each of the nouns is followed by the particle に (ni), which ties it to the verb (similar to the “into” in “change into”), and then the particle も (mo), “also.” In this case, the doubled も…も followed by a negative ending means that the best rendition is probably “neither… nor.”
Some versions of this phrase may use an older form of the negative ending: ならぬ (naranu).
This kotowaza comes from Ejima Kiseki (江島其磧)’s 1710 ukiyozoushi novel 『けいせい伝受紙子』 (Keisei Denju Gamiko).
(“A, Kenta-kun? Tsuite kuru kedo ki ni shinakute ii nda yo. Yaku ni tatanai kedo jama mo shinai, doku ni mo kusuri ni mo naranai yatsu da.”)
[“Oh, Kenta? He’ll follow us around, but don’t let it bug you. He doesn’t do any good but he doesn’t get in the way either; he’s just kind of there.”]