1. Let’s go to the beach. You don’t want to sit around the house all day, do you? Don’t be such a stick-in-the mud!
2. Everyone thought Herman was a stick-in-the-mud because he never liked to do anything interesting or exciting. He never took part in the tricks that the other students played on each other or on their teachers.

Compare to: wet blanket. The expression stick-in-the-mud is derogatory. It is usually said to taunt someone who is less daring, less spontaneous, or more conservative than others. It suggests the slow or sluggish movement of something stuck in mud.