IPA: /ˈhjuːmərləs/
KK: /ˈhjuːmərləs/
Not having or showing a sense of humor; serious or dull.
His humorless attitude made it difficult for others to enjoy the party.
Humorless → It is formed from "humor" (from Latin *humor*, meaning fluid or moisture, which evolved to mean a mood or temperament) and "-less" (meaning without). The word "humorless" describes a state of being without humor or lacking a sense of fun.
Think of someone who is 'without' ('-less') a 'mood' or 'temperament' ('humor') — that's why humorless means lacking humor.