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.
Possible explanations:
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.
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