A baby, used as an insult. The insult seems to pre-date Skyrim, and may have origins in Scottish culture.
An old Scottish saying is:
"Bread and cheese is gude to eat when folk can get nae ither meat.
Bread and milk is bairns' meat: I wish them sorrow that be it."
Seems to refer to a "milk drinker".
Another Example: "What do you think of that, Mr. Pajama-Wearing, Basket-Face, Slipper-Wielding, Clype-Dreep-Bachle, Gether-Uping-Blate-Maw, Bleathering, Gomeril, Jessie, Oaf-Looking, Scooner, Nyaff, Plookie, Shan, Milk-Drinking, Soy-Faced Shilpit, Mim-Moothed, Sniveling, Worm-Eyed, Hotten-Blaugh, Vile-Stoochie, Cally-Breek-Tattie?"
An insult / derogative term deriving from the video game Skyrim, often used by City Guards to suggest a person is weak and / or young / inexperienced.
Young babies drink milk from their mother's breasts.
and / or
Those who can't handle mead or ale instead drink milk.
and / or
Milk in Skyrim will restore a Warrior's resource for hard-hitting sword swings. Those who require Milk for this purpose would likely be lower-level, and inexperienced.
Don't be such a milk drinker.