Say it in kana
I found an interesting entry in Maeda Isamu's Edogo no jiten (江戸語の辞典, "Dictionary of Edo-period language"): kana de iu, literally "say it in kana". This could mean either "say it in simple language" or "say it without beating around the bush", according to Maeda.
His example sentence is from the 1779 Ekisha san'yū: "Iya nara iya to kana de iinanshi, iya tomo ō tomo iikirinanshi", "If you won't, just say so in kana; yes or no, out with it." I suppose this matches English expressions like "in plain English" quite closely, but with an extra twist in the blurring of the boundary between written and spoken language. (What the expression kana de iu calls to mind, for me, isall those Edo-period illustrated stories for reg'lar folks, where the dialogue is — indeed! — all or nearly all in kana, with kanji reserved for the kanbun preface and so on, although of course you can trace a similar split back all the way to the early all-kana monogatari vs the all-kanji documents of administration and so on.)
It's hard to blame all those mutton-chopped orientalists for having concluded that East Asians think in "ideograms" and so on, when you run into things like this.