lettuce: [13] The lettuce was named for the milky white sap that exudes from its stalk when cut. The Latin for milk is lac (source of English lactation [17] and lactose [19]), and so the lettuce was christened lactūca. This passed into English via laituës, the plural of Old French laituë. => lactation, lactose
lettuce (n.)
late 13c., probably from Old French laitues, plural of laitue "lettuce," from Latin lactuca "lettuce," from lac (genitive lactis) "milk" (see lactation); so called for the milky juice of the plant.