Casual slang describing something or someone as uninteresting, flat, or lacking energy. Often used in youth speech.
This movie is seriously boring.
Today's class was so dull.
I feel kind of lifeless today.
I watched a boring movie.
だるい is a native Japanese adjective meaning physically sluggish or lethargic, whereas ダル is a separate slang loanword meaning 'boring' or 'lifeless' about a thing or situation.
dull game
Loanword from English 'dull', used as casual slang in Japanese.