From Longman Dictionary of Contemporary Englishbacklessback‧less /ˈbækləs/ adjective a backless dress, swimsuit etc does not cover much or any of a woman’s back
Examples from the Corpus
backless• I waited, sitting as quietly and unobtrusively as possible on the low backless benches in front of the ground-floor lobby windows.• She would have turned heads anywhere in a navy blue backless dress which rose just above her knee.• There, in the smoky crowd, you might encounter Marlene Dietrich in a backless gown or the novelist Sybil Bedford.• Although I doubt even Camilla would wear the backless Lycra frock that this woman almost had on.• To go with it she wore a turquoise-and-gray-striped apron and matching backless slippers.• The black taffeta silk ballgown was strapless and backless with a plunging, gravity-defying decolletage. 