There have been numerous attempts to have the use of the word milk restricted to “a fluid secreted by the mammary glands of females for the nourishment of their young,” at least when labelling a product intended to be consumed.
This much is undisputed: Soybeans do not lactate. So soy producers shouldn't be calling their beverages "milk," according to the National Milk Producers Federation, which filed a complaint with the Food and Drug Administration in February seeking to banish the terms "soy milk" and "soymilk" from grocery shelves and dairy cases.
— Cindy Skrzycki, Austin American-Statesman, 12 Mar. 2000
We have the ‘mammary secretion’ definition of milk as the oldest sense of the word, but it is by no means the only one which applies to liquid. We also define milk as “a food product produced from seeds or fruit that resembles and is used similarly to cow's milk” and “a liquid resembling milk in appearance: such as the latex of a plant.” We have been referring to a liquid formed with ground almonds as almond milk since the 15th century, and milk has been used to describe the secretion of some plants well before that.
Also it is good agaynst the cough yf … be soden in water of barley with Almond mylke and penycles put therto.
— Anon., _The grete herball, 1526