un·der·dress 
/ˌʌndɚˈdrɛs/ verb
un·der·dress·es; un·der·dressed; un·der·dress·ing /ˌʌndɚˈdrɛs/ verb [no obj] : to dress in clothes that are too informal or not warm enough for an occasion
▪ I don't want to underdress for the party. — opposite overdress
▪ I don't want to underdress for the party. — opposite overdress
— underdressed adjective
▪ They were underdressed for the wedding. ▪ I was underdressed for the weather.
▪ They were underdressed for the wedding. ▪ I was underdressed for the weather.
