un·der·dress 
(ŭndər-drĕs′)
(ŭndər-drĕs′)v. (ŭn′dər-drĕs) un·der·dressed, un·der·dress·ing, un·der·dress·es
v.intr.
1. To dress oneself too informally for an occasion: Don't underdress for the wedding; blue jeans are not appropriate attire.
2. To dress oneself too lightly: When it began to snow, I realized I had underdressed for the hike.
v.tr.
1. To dress too informally for an occasion: Seeing the other guests in their evening gowns made me feel I had underdressed myself.
2. To dress too lightly: You're underdressing the baby for such a cold day!
n.
1. Apparel worn beneath outer garments; underclothing.
2. An outer garment, such as a dress beneath a tunic or coat, that is worn as part of a costume or suit.