While the term 'Muggle' is not in popular usage anymore, it once used to be a term that was informally synonymous with the term 'sweetheart'. However, its introduction in the Harry Potter universe has made it a commonly used word in the 21st century. The term 'Muggle' is now used to refer to someone who is not particularly skilled in a field or discipline, just like the Muggles in the Harry Potter universe who couldn't do magic.