The business end of a butter churn plunger: a disk with holes in it, attached to a staff.

Plunge, or churning staff

Inked and painted illustration of the inside part of a butter churn, the plunge: it is a stick with a disk attached to it, with holes in the disk.  It is disk-end up.
Plungy!

There are a couple of folks in the “Unofficial Heraldry Chat” group for the SCA who regularly post “not yet registered” things they’ve found in period armory. Recently, they posted the actual working part of a butter churn — not the tub, with which I think most people are familiar, but the thing inside that you use to actually churn the butter: the plunge, or churning staff.

Looks like the incomparable Iago ab Adam already beat me to it in terms of making something for the Book of Traceable Heraldic Art, but my somewhat more rough/drawn-looking one is exemplar #2. It is from the State Archives of Austria, from the “von Sonnberg” arms, possibly drawn or compiled by one Heinrich Neumayr (but my German is super rusty, so I’m probably wrong).