Definition of mourning widow in US English:

mourning widow

noun

  • Either of two plants with deep purple flowers, both native to Europe and cultivated elsewhere: (a) dusky cranesbill, Geranium phaeum, of central Europe.

Origin

Early 19th century (in an earlier sense). From mourning + widow.

Pronunciation

mourning widow

/ˌmɔːnɪŋ ˈwɪdəʊ/