Yes, plain is a Scrabble word!

Adverb

1. Unmistakably (`plain' is often used informally for `plainly')

Verb

1. Express complaints, discontent, displeasure, or unhappiness

Adjective

1. Clearly revealed to the mind or the senses or judgment

2. Not elaborate or elaborated; simple

3. Lacking patterns especially in color

4. Not mixed with extraneous elements

5. Free from any effort to soften to disguise

6. Lacking embellishment or ornamentation

7. Lacking in physical beauty or proportion

Noun

1. Extensive tract of level open land

2. A basic knitting stitch