Latin lactuca was named from lac, milk, because of the plant’s milky juice. The word travelled through French and Middle English before the modern spelling lettuce.
The source says French letuse probably comes from the plural of letue.
Sources
- Merriam-Webster: lettuce
Word History gives lac/lact-, lactuca and the French/English forms, with a qualification on the French plural development.
- logeion.uchicago.edu · lac
Meanings of the older forms on the word path.
- Online Etymology Dictionary · lettuce
Meanings of the older forms on the word path.
- logeion.uchicago.edu · lactuca
Meanings of the older forms on the word path.