Etymon
pie-ne-protoindoeuropean-n-protogermanic-un-protowest-germanic-un-old-english-un-middle-english-un-english-un-protogermanic-hrewwana-protowest-germanic-hreuwan-protoindoeuropean-h2-protoindoeuropean-eh2-protogermanic-o-protowest-germanic-u-protowest-germanic-hreuwu-old-english-hreow-middle-english-rewe-protoindoeuropean-pleh1-protoindoeuropean-nos-protoindoeuropean-plh1nos-protogermanic-fullaz-protogermanic-fullaz-old-english-ful-middle-english-ful-middle-english-reuful-english-rueful-english-unrueful-from-un-rueful
A dedicated page for this root is on the way — it will gather every word descended from it, sibling forms across daughter languages, and a branching map of the family tree.