Английский Этимологический словарь - manure (v.)

- 14c., from Anglo-Fr. meynoverer, from O.Fr. manouvrer "to work with the hands," from M.L. manuoperare, from L. manu operari, from manu, abl. of manus "hand" + operari "to work, operate." Sense of "work the earth" led to "put dung on the soil," which led to the current noun meaning, first attested 1549.