adjective

 

simple simple and not complicated

plain food

 

not mixed not mixed with other colours, substances, etc

a plain blue carpet

 

person A plain person is not attractive to look at.

 

obvious obvious and clear

It's quite plain that she doesn't want to talk to me about it.

 

noun

 

a large area of flat land

 

adverb

 

completely stupid/wrong, etc

That's just plain stupid!