D defs.my
Entry 1 sense Webster, 1913

Copartner

/kōp'-ärˌ-tnẽr/ · Co·part·ner · IPA /kˈoʊpˌɑrtnər/
01 n. One who is jointly concerned with one or more persons in business, etc.; a partner; an associate; a partaker; a sharer.
  1. 1.
    One who is jointly concerned with one or more persons in business, etc.; a partner; an associate; a partaker; a sharer.
    “the associates and copartners of our loss.” Milton.