後の祭り/the opportunity has passed; regret something as too late

祭りが終わったあとの山車や祭祀用具の意から、時機が過ぎて役に立たないことのたとえ。また、後悔しても手遅れのことをいう。

Comes from the idea of the float and ritual items being left over after the festival. An example of the opportunity having passed and things that are not useful being left over. Regret at being too late.

山車・だし・float
祭祀・さいし・rite, ritual
時機・じき・chance, time of an opportunity
後悔・こうかい・regret
手遅れ・ておくれ・too late

虻蜂取らず/Can't catch the horsefly nor the bee

虻と蜂を一度に追いかけて、両方とも取り逃がしてしまうという意。両方のものを得ようとして、どちらも取り逃がすこと。欲張りすぎたために、かえってだめになることのたとえ。
Comes from the idea that if you chase after both a fly and a bee at one time, you .won't be able to catch either and they will both run away. If you try and get two things, you'll miss catching both. An example of being too greedy and hence all the better that it became bad.

「仕事と趣味を両立させようなんて、結局、虻蜂取らずに終わるぞ」

虻・あぶ・horsefly
取り逃がす・とりにがす・miss, lose, fail to catch x wo - 他動詞
欲張り・よくばり・to be greedy, or a greedy person
両立・りょうりつ・compatible

油を売る/sell oil

仕事中に時間をつぶして怠けること、また仕事中に長々とむだ話をすることをいう。　　「いつまでも油を売っていないで、早く職場に戻れ」。
Waste time and neglect things while you are at work. Alternatively, while at work talk a long time about meaningless things. "Dont spoend any time selling oil, return quickly to your workplace".

江戸時代の油売りが客の容器に油を移すのび時間がかかったことからとも、髪油を売り歩く商人が婦女を相手にのんびりと話し込みながら商売したからともいわれる。
In the Edo period it took a long time to transfer oil to the containers of the customers, so salesmen who sold hair oul would spend a long time in relaxed conversation with housewives.

怠ける・なまける・be lazy
長々と・ながながと・for a long time
婦女・ふじょ・woman