It is an adjective and it means "to feel sluggish". It is used when you feel very heavy and dull because you are tired or sick. It is also used when you feel that something is a hassle.
It is similar to "面倒くさい". "だるい" is more focused on your physical condition and it indicates that you have less energy to do something. "面倒くさい" is more focused on your mental condition and it indicates that you don't want to do something.
I studied and concentrated well yesterday, but today I'm not in the mood for studying for some reason.
昨日はかなり集中して勉強したけど、今日は何かだるい。
昨日はかなり集中して勉強したけど、今日は何かだるい。
Amy is packing some stationeries because it has to be shipped by 5 pm.
- Meg : Hey, have you finished packing yet?
もう荷造り終わった？ - Amy : No. I have a lot of stuff to do, but I'm not really making much progress with my work today. I don't feel well.
まだ。やる事がいっぱいあるけど、全然進まない。なんか、だるい。 - Meg : You've been busy with work lately. Are you tired? Or did you catch a cold?
最近忙しいからね。疲れた？それとも、風邪でも引いた？ - Amy : I'm not sure but I feel sluggish and I might have a fever. I'm going to finish this and leave early today.
よく分からないけど、だるくて、熱があるかも。これを終わらせて、今日は早く帰るね。
Ken and Leo are talking about their plans for the weekend.
- Leo : What are you going to do this weekend?
週末は何をするの？ - Ken : I just wanted to relax, but I'm moving on Sunday. I have to finish packing and then clean my whole house tomorrow. Oh, it's a hassle!
ゆっくりしたかったんだけど、日曜に引っ越しをするんだ。明日は、荷造りして、家中掃除しなきゃ。あー、だるい。