underrunnerEtymologyFrom under- + runner.nounThe lower of a pair of grinding millstones.“FOR SALE AT A BARGAIN. […] Four run stones, Munson underrunners, 3 ft diameter.”An additional runner added to the bottom of a sledge to protect the main runners in harsh conditions.“Less than half a mile to the south the party sighted an underrunner of a sledge, and assumed it to mark the location the Norwegians had assigned for 90°S.”A runner (mechanical part intended to guide or aid something else to move) designed to be inserted underneath the item to be moved.“A triple tandem tow using underrunners. This method has been used for long ocean tows by tugs with a single-drum winch.”