Other forms: plainer, plainest, plains
Plain means nothing has been added — no pattern, no decoration, no flavouring. A plain white shirt, plain yoghurt, a plain wooden table with nothing standing on it.
Plain also means easy to see or understand: it was plain that she had been crying, plain English, hidden in plain sight. Of a person it means direct and honest — plain speaking, a plain answer — but of a face it is an unkind word for not pretty, so keep it for shirts. The noun is a different picture altogether: a plain is a wide flat stretch of land with almost nothing on it, and the famous ones come in the plural, the Great Plains. And there is the trap every learner meets, because plane sounds exactly the same and means an aircraft or a flat surface: plain rice, a plane ticket.
1adjectiveplainWith no pattern, decoration or added flavour.~26%
“She wore a plain black dress and no jewellery.”
2adjectiveclearEasy to see or to understand.~19%
“It was plain that nobody had read the report.”
synonyms: clear, obvious
3adjectivefrankDirect and honest, sometimes bluntly so.~2%
“He gave me a plain answer for once.”
synonyms: frank, blunt
4nounlandA wide flat stretch of open land with few trees on it.~53%
“The road ran straight across the plain for fifty miles.”
Shares are an estimate: a language model read 169 sentences found in the wild and placed 141 of them on a sense (28 were unclear) — a count of our sample, not a measurement of English.
Latin plānus meant flat, even, level, and English ended up with it several times over: plain, plane and plan are the same word arriving by different roads, and so is piano, which came through Italian. Flatness is the thread through every one of them — flat land, a flat surface, a sheet with the drawing laid out on it, and a shirt with no pattern raised on it.
Proto-Indo-European
*pleh₂-
Latin
plānus — flat, even, level, plain
Old French
plain
Middle French
plain
Anglo-Norman
pleyn
Middle English
pleyn
English, today
plain
From Middle English pleyn, borrowed from Anglo-Norman pleyn, playn, Middle French plain, plein, and Old French plain, from Latin plānus (“flat, even, level, plain”). a doublet of llano and piano and plane and plan.
Etymology from Wiktionary, CC BY-SA