A stirrup is a footrest for horse riders, a flat-bottomed ring of metal hung by a strap from the saddle. The standard heraldic form is the English stirrup; it is sometimes so blazoned. It’s a period charge, dating from 1308 in the arms of Kydemore [ANA2 549].
In medieval heraldry, the stirrup was usually drawn “leathered”, i.e., showing a bit of strap, though there are examples of unleathered stirrups as well (e.g., the badge of Gyfford, c.1520 [Walden 201]); in Society heraldry, the leather is not shown unless blazoned. The Society also has a single example of a “stirrup-cup”, which is identical to an inverted escutcheon in shape; this charge is unattested in period armory. See also spur.
The Order of the Golden Stirrup, of Æthelmearc, bears: A stirrup Or leathered gules.
Berengaria de Hainault bears: Azure, three stirrups Or, leathered argent.
Meadhbh inghean ui Bhaoighill bears: Per fess azure and Or fretty azure, a stirrup Or.