"vessel in which cream or milk is agitated to separate it and make butter," Old English cyrin, from Proto-Germanic *kernjon (source also of Old Norse kirna, Swedish kärna, Danish kjerne, Dutch karn, Middle High German kern); probably akin to cyrnel "kernel" (see kernel) and describing the "grainy" appearance of churned cream.
mid-15c., chyrnen, "to stir or agitate (milk or cream) to make butter," from churn (n.). Extended sense "shake or agitate violently" is from late 17c. Intransitive sense is from 1735. Related: Churned; churning. To churn out, of writing, "produce mechanically and in great volume" is from 1902.