A stock phrase in Japanese pop culture supposedly used by the wife in a newly-wed couple to invite her new husband to bed.

Kyūjitai form of 私生児 (“illegitimate child”)

“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