@Sorrowless: It's not a literal term
Throughout most of history and for most animals and even a large percentage of humans today, nothing drinks milk after being weaned of it. Drinking milk is thus a metaphor for juvenility.

@Fenrick: wait, so do those cookies and milk I just had make me juvenile?

I should mention I was also watching cartoons.

Most of the west handles lactose pretty well so it's still an insult. Scandinavians also happen to be the most lactose tolerant in the world so I've heard. It's a good possibility that Nords share that ability.