A churn is a vessel in which milk is agitated, usually with a plunger, to make butter. It was not a common charge, but an example is found in the badge of Alchorne, 15th C. [Siddons II.2 331]. The illustration is taken from the book of hours by the Master of James VI of Scotland, c.1510.
Marion FitzWilliam bears as a badge: Argent, in pale a lobster fesswise contourny gules and a wooden butter churn enflamed proper and a chief wavy azure.