- a small rivulet or brook.
"small brook, rivulet," 1538, from Du. ril, Low Ger. rille "groove, furrow, running stream," probably from P.Gmc. *riðele (cf. O.E. rið, riþe "brook, stream," which survives only in obscure Eng. dialects), a diminutive form from PIE base *reie- "to run, flow" (see Rhine).
"My landscape gardeners incorporated all the essential features, I believe. They spent simply ages getting the rills sufficiently sinuous."
- Terry Pratchett, Sourcery