The phrase "strapless cocktail" refers to a style of dress that does not have shoulder straps and is specifically designed to be worn for formal or semi-formal occasions, such as a cocktail party. Full definition
Numbered Scherrer black chiffon strapless cocktail dress with chiffon overlay. (1stdibs.com)
Stunning 1980s Victor Costa black strapless cocktail dress. (1stdibs.com)
«A Peter Soronen strapless cocktail dress... silver and black, cinched waist, above the knee adorned with fine mesh chains draping from the neckline.» (hourdetroit.com)