[proverb] “a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds
(This term, ひゃくにちのせっぽうへひとつ(hyakunichinoseppōhehitotsu), is the hiragana spelling of the above term.)