I doubt there are many animals that drink milk from other species, but it seems that we have been doing so for a very long time. The word itself is certainly ancient.
If we go back to Middle English, we can find several different forms of the word. It was variously spelt milc, meolc, melc, melk and even the familiar milk. Not much change from that period up until the present day, it would seem.
There are still regions today where the vowel is pronounced more like /e/ than the standard /i:/ . Let’s take a look at an example from 1398. It has been taken from Trevisa’s translation from Latin into English of Bartholomaeus’s De Proprietatibus Rerum, or On the Properties of Things:
Ivy..multiplieþ melk in geet.
(Ivy increases milk in goats).
Here, it has been written as melk. This version of the word is closer to the Old English forms meoluc and meolc and occasionally meoloce. Let’s take a look at an example from Old English:
Gáte geallan meng wið cú-meoluc
(Mingle goat’s gall with cow’s milk)
This is from Leechdoms, Wortcunning, and Starcraft of early England, a collection of writings published in the 1800s, dating back to the late ninth century. Another example, this time from 1072, is from the Old English text Codex Exoniensis (or Exeter Book):
Mid lytle meolc wætere gemengedre
(With a little milk mixed with water)
So you can see that the word has not changed a great deal since that time. But where did it come from? Well, it has a number of cognates in other Germanic languages, such as German Milch, Dutch and Norwegian melk, Danish mælk, Swedish mjölk (from Old Norse mjolk). These have helped to trace the word back to Proto-Germanic and *meluk, with the same meaning as today’s word.
We can take a further step back in time and trace *meluk to the Proto-Indo-European root *melg-, meaning ‘wipe, rub off, stroke’. This would have developed in sense through the hand motion needed when milking an animal. It has a modern cognate today in Greek; αρμέγω [armego], which means ‘to milk (an animal)’.
It is also cognate with Latin mulgere, which meant ‘to milk’, and this is an interesting connection, as ‘emulsion’ is derived from mulgere, via French into English. This means that milk and emulsion are distant cousins!