stick-in-the-mud

/S T IH1 K IH0 N DH AH0 M AH0 D/
noun
  1. 1

    A person unwilling to participate in activities; a curmudgeon or party pooper.

    โ€œHave a little fun sometimes and don't be such a stick-in-the-mud.โ€

  2. 2

    More generally, one who is slow, old-fashioned, or unprogressive; an old fogey.

Source: https://en.wiktionary.org/wiki/stick%20in%20the%20mud, https://en.wiktionary.org/wiki/stick-in-the-mud