百日の説法屁一つ: “a hundred days of sermonizing, one fart” → a metaphor for how extreme effort 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

(chiefly literary) truly; indeed; without doubt

to preach

sermon, lecture

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

long / (time) long, lengthy

chief, head, leader

strong point / (music) major / chief, head, leader