It's an adjective and it means "to be ridiculous". It is used when you feel that something is stupid or unreasonable, and it's worthless. It's interchangeable with "馬鹿らしい". You may hear its synonym "アホらしい" or "アホくさい".
Meg is browsing Amy's horoscope.
- Meg : You should go out with Ken. If I remember correctly, he is a Pisces. It says that you are compatible with Pisces. What do you say?
ケンと付き合いなよ。たしか、彼は魚座だよ。エイミーは魚座と相性がいいんだって。どう？ - Amy : Huh? How many male Pisces do you think there are in the world? I don't believe in horoscopes. They’re absurd.
えっ？世界中に魚座の男子が何人いると思う？星占いなんて信じないよ。馬鹿馬鹿しい。
Ken bought a cup of coffee at a new fancy cafe and took it to the office.
- Leo : Oh, you got the coffee at that cafe. I've wanted to give it a try, but it looks expensive.
あっ、例のカフェで、そのコーヒーを買ったんだ。俺も試してみたかったんだけど、高そう。 - Ken : It costs more than 10 dollars.
10ドル以上したよ。 - Leo : Is it worth it?
高いだけあって、上手い？ - Ken : Let me see...(He drinks it) Hmm, there’s no difference from the one I always buy. Ugh, that's ridiculous.
どれ...んー、俺がいつも飲んでいるのと変わらないな。ふんっ、アホくさ。
Meg and Amy are working together.
- Meg : I'd like Tom to help me, but I haven't seen him since this morning. Do you know where he is?
トムに手伝ってほしいんだけど、今朝から見かけないね。どこにいるか知っている？ - Amy : Yeah. He called me and told me that he overslept and will be here later this afternoon.
うん。電話があって、寝坊したから午後に来るって。 - Meg : Again? I can't believe that we get paid the same amount of money.
また？(私達が彼と)同じ給料だなんて、信じられない。 - Amy : I know. I feel stupid for taking this job seriously!
確かに。真面目に働くのが馬鹿馬鹿しいね。