c. 1400, "to cultivate (land, a garden) by manual labor," also "to hold property, rule," from Anglo-French meynoverer (late 13c.), Old French manovrer "to work with the hands, cultivate; carry out; make, produce," from Medieval Latin manuoperare (see maneuver (n.))
Sense of "work the earth" led to "put dung and compost on the soil, treat (soil) with fertilizing materials" (1590s) and to the noun meaning "dung spread as fertilizer," which is first attested 1540s. Until late 18c., however, the verb still was used in a figurative sense of "to cultivate the mind, train the mental powers."
It is ... his own painfull study ... that manures and improves his ministeriall gifts. [Milton, 1641]
Related: Manured; manuring. Another Middle English word for "manuring" was donginge.