Definition

In the Harry Potter universe, ordinary non-magical folk blissfully unaware that wizards are living among them. The term has transcended its fictional origins to describe anyone oblivious to a particular subculture or specialized knowledge.

Example Usage

We can't talk about the surprise party in front of Sarah—she's a muggle when it comes to our plans.

Source: Urban Dictionary (https://www.urbandictionary.com/define.php?term=muggles&defid=12053)