(Nodomoto sugireba atsusa wo wasureru; “To forget the heat after swallowing”)
No matter how painful a situation was, the pain – or alternately, the debts owed to people who helped out when times were bad – are quickly and easily forgotten once the situation is over. The image is of a hot drink (or food) that burns your mouth, but no longer bothers you once it’s been swallowed.
We begin with the noun 喉元 (nodomoto), “the base of the throat,” and then the verb 過ぎる (sugiru), “to go past,” in conditional form. (This may feel odd grammatically because the thing performing the verb is presumably the hot drink, which doesn’t explicitly appear in the sentence.) After this we come to the main clause. The particle を (wo) marks the noun 熱さ (atsusa), “heat,” as the object of a verb, and the verb itself is 忘れる (wasureru), “to forget,” in sentence-final form.
Replacing 熱 with homophone and near-synonym 暑 is an error: the former refers to the heat held within a physical object, while the latter is ambient heat in the air.
This is the の entry of the Edo iroha karuta set.
「勇介くんは何人にフラれてもすぐ別の女の子に惚れて告白しちゃうね」 「うん、喉元過ぎれば熱さを忘れる人ね」 「そうかな。ただの信頼のできないバカなプレイボーイじゃない？」
(“Yuusuke-kun wa nannin ni furarete mo sugu betsu no onna no ko ni horete kokuhaku shichau ne.” “Un, nodomoto sugireba atsusa wo wasureru hito ne.” “Sou ka na. Tada no shinrai no dekinai baka na pureibooi ja nai?”)
[“No matter how many people have turned him down, Yuusuke turns around and falls for another girl.” “Yeah, he’s one of those people who forget their pain as soon as it’s passed.” “Maybe. Or is he just an unreliable idiot playboy?”]