underdressed

verb (used without object), un·der·dressed or un·der·drest, un·der·dress·ing.

1. to clothe oneself less completely or formally than is usual or fitting for the circumstances.

noun

2. garments worn beneath others; underclothes.
3. a slip, petticoat, or other underskirt, especially one designed to be seen when worn, as beneath a redingote.

adjective

1. wearing clothes that are not elaborate or formal enough for a particular occasion

adj.also under-dressed, “too plainly dressed,” 1759, from under (adv.) + past participle of dress (v.).