Plain
As an adjective
Specifically meaning clear, straight to the point.
Aboveboard
Abrupt
Artless
Bluff
Blunt
Brusk
Brusque
Candid
Curt
Direct
Ernest
Forthright
Foursquare
Frank
Freespoken
Gruff
Honest
Impolite
Inconsiderate
Open
Openhearted
Outspoken
Plainspoken
Pointblank
Rude
Sharp
Sincere
Straight
Straightforward
Tactless
True
Truthful
Undiplomatic
Guarded
Uninhibited
Unreserved
Unrestrained
Veracious