One way to hold plainly: picture flat open ground with nothing standing on it. Nothing has been put in front of you, so you see and hear the thing whole. Nothing has been wrapped around the words, so the truth arrives whole. And nothing has been added on top for show, so what is left looks simple. Same empty ground, three different things missing from it.
From the word’s history
The evidence packet's Wiktionary entry traces plainly to Middle English playnly, that is plain plus -ly. The adverb has carried the adjective's picture of open, unadorned ground into all three modern uses.
證據包的 Wiktionary 記載 plainly 來自中古英語 playnly，也就是 plain 加上 -ly。這個副詞把形容詞那片「空曠、沒有裝飾」的畫面，一路帶進了今天的三個用法裡。
Wiktionary entry for plainly, kaikki extract supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:987121408:5794)
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