A “stick in the mud” is someone described as dull, unwilling to try new things, or old-fashioned. The label is disapproving, but friends may use it jokingly when someone is unusually quiet or not joining in. The story also distinguishes a “fuddy-duddy,” who resists change, from a “stuffed shirt,” who is overly serious.
Cultural note: This is a negative label, even when said playfully. A quiet person may be tired, unwell, or dealing with private news; it is kinder to ask how they are before assuming they dislike fun. “Fuddy-duddy” sounds playful and old-fashioned, while “stuffed shirt” criticizes a pompous or overly formal manner.
Keep exploringThe phrase pictures someone stuck and unable to move—but in conversation, pause before deciding why a person is sitting out.