From under- + runner. <span class="searchmatch">underrunner</span> (plural <span class="searchmatch">underrunners</span>) The lower of a pair of grinding millstones. 1890, The Northwestern Miller, volume 29, page 563:...
<span class="searchmatch">underrunners</span> plural of <span class="searchmatch">underrunner</span>...
volume 29, page 563: FOR SALE AT A BARGAIN. […] Four run stones, Munson <span class="searchmatch">underrunners</span>, 3 ft diameter. […] All with idlers and in the best condition. (lazy...
the runner stonerunner torch runner tree runner treerunner ultrarunner <span class="searchmatch">underrunner</span> wave-runner wave runner waverunner somebody who runs, who moves at a...