From Longman Dictionary of Contemporary Englishstraplessstrap‧less /ˈstræpləs/ adjective → strapless dress/gown/bra
Examples from the Corpus
strapless• The black taffeta silk ballgown was strapless and backless with a plunging, gravity-defying decolletage.• I saw a strapless black gown in the corner, a pair of stilettos discreetly hidden on the floor.• Susanna wore a black silk strapless dress.• I had a little dress made - it was a strapless taffeta number, very girly and aqua blue. 