always a bridesmaid, never a bride

Etymology
Title of a 1917 English Music Hall song by Charles Collins. Later used in a United States advertisement for Listerine mouthwash (implying that the use of mouthwash could improve a woman's chances of marriage).

Usage notes
Also commonly used more literally, of a woman who assumedly has acted as bridesmaid at weddings but has not yet married. An assumption of personal desire to marry is implied – as is, in general terms, the desirability of marriage. Use of the phrase may constitute or be construed as social pressure to marry.