Apparel worn beneath outer garments; underclothing.
An outer garment, such as a dress beneath a tunic or coat, that is worn as part of a costume or suit.
To dress too informally for an occasion.
Remember not to underdress for the wedding.
To dress (oneself, for example) too informally for an occasion.
You don't want to be underdressed at the awards ceremony.
To dress more plainly or informally than is indicated by the occasion.
Origin of underdress
From under- +"Ž dress.