Kindred traces back to Middle English kinrede (also spelled kinraden), built from kin — family, tribe, people related by blood — plus -rede, a descendant of Old English rædan, meaning roughly "condition" or "state of being." That same ending shows up in "hatred," which literally breaks down as "the state of hating." So structurally, kindred just means "the state of being kin." Sometime around the 1600s, English speakers slipped an extra "d" sound into the middle — probably because "kinred" was awkward on the tongue, a change likely nudged along by kindred sounding so much like it belonged next to the word "kind."

For its first few centuries, kindred was strictly a noun — your kindred were simply your relatives. The adjective use we lean on today (a kindred spirit, a kindred soul) didn't show up until the 1520s. And the phrase's cultural staying power owes a real debt to literature: in L.M. Montgomery's 1908 novel Anne of Green Gables, Anne Shirley declares that "kindred spirits are not so scarce as I used to think" — a line so quotable that readers have been borrowing it to describe their own soul-matching friendships for over a hundred years.