colloquial form of 'tsumaranai' meaning boring or dull; used in casual speech.
昨日の映画、つまんないよ。
Yesterday's movie was boring.
そんなこと言っても意味ないよ。つまんないこと言わないで。
Saying that won't help. Don't say boring/stupid things.
雨で外に出られなくて、一日中つまんない。
It was raining so I couldn't go out — it was boring all day.