"plainest" in the adjective sense
1. apparent, evident, manifest, palpable, patent, plain, unmistakable
clearly revealed to the mind or the senses or judgment
"the effects of the drought are apparent to anyone who sees the parched fields"
"evident hostility"
"manifest disapproval"
"patent advantages"
"made his meaning plain"
"it is plain that he is no reactionary"
"in plain view"
"a palpable lie"
2. plain
not elaborate or elaborated simple
"plain food"
"stuck to the plain facts"
"a plain blue suit"
"a plain rectangular brick building"
3. plain, unpatterned
lacking patterns especially in color
4. plain, sheer, unmingled, unmixed
not mixed with extraneous elements
"plain water"
"sheer wine"
"not an unmixed blessing"
5. plain, unvarnished
free from any effort to soften to disguise
"the plain and unvarnished truth"
"the unvarnished candor of old people and children"
6. plain, bare, spare, unembellished, unornamented
lacking embellishment or ornamentation
"a plain hair style"
"unembellished white walls"
"functional architecture featuring stark unornamented concrete"
7. homely, plain
lacking in physical beauty or proportion
"a homely child"
"several of the buildings were downright homely"
"a plain girl with a freckled face"
Source: WordNet® (An amazing lexical database of English)
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