1640s, "allied by blood, connected or related by birth, of the same parentage, descended from a common ancestor," from Latin cognatus "of common descent" (source also of Spanish cognado, Italian cognato), from assimilated form of com "together" (see co-) + gnatus, past participle of gnasci, older form of nasci "to be born" (from PIE root *gene- "give birth, beget").
Of things, "related in origin, traceable to the same source," by 1640s; specifically of words, "coming from the same root or original word but showing differences due to subsequent separate phonetic development," by 1782; of languages, "from the same original language," by 1799. French, Spanish, and Italian are cognate languages (all essentially descended from Latin) but are not cognate with Latin. English cognate, Spanish cognado and Italian cognato are cognate words from Latin cognatus. English brother, Sanskrit bhrtr-, Greek phratr, Latin frater, Russian brat are cognate words from the PIE root *bhrater. Words that are cognates are more like cousins than siblings; they develop in different languages.
Related: Cognatic; cognation (late 14c. in English as "blood-relationship, kinship"); cognateness. As a noun, "one connected to another by ties of kinship," from 1754.