退屈
たいくつ (taikutsu)
boring, boredom
na-adjective/noun
Example
雨の日は家にいて退屈です。
あめのひはいえにいてたいくつです。
Ame no hi wa ie ni ite taikutsu desu.
On rainy days, I'm bored staying at home.
Can be used as both a noun (退屈な人 - boring person) and na-adjective (退屈な映画 - boring movie). Very useful for expressing feelings about activities or situations.