退屈/たいくつ (retrat, resign + yield, submit) = tedious, boring, dull, uninteresting... not my favorite but I find it hard to relate the meaning of those kanji to the idea of boredom...
I think it’s easier to remember 退屈 as bored first, retreating and flinching from one’s duties. I wrote a mnemonic to help.
It certainly qualifies as a Japanese idiom, in my opinion. It’s an original Japanese word, and Kotobank has attestations as far back as the tenth century, which is about as old as written records can get.
