You learn something new everyday! Good to know now
it's a type of paint that is pigment suspended in gum arabic or similar
this is another similar one, that is made from milk:
https://en.wikipedia.org/wiki/Milk_paint
both of these paint types are like watercolour in that they are water based but can carry a lot more pigment so they are more rich and opaque and can be layered more, and if the pigment is zinc oxide or titanium dioxide you can even fully mask over top of other paintwork
Discussion
yeah, i could have sworn that back in the olden days gouache was made from milk too but there ya go