Describes a face or surface that is completely plain, lacking distinct features or expression. Often used for a face without makeup or with a blank expression.
See also: ずべら坊
She has a plain, featureless face without makeup.
That wall is plain and smooth, with nothing on it.
Describes a person or appearance that is careless, unkempt, or slovenly. Less common than the 'plain' sense; often used in a derogatory way.
He came to work looking slovenly.