Mid 19th century. Short for post-classical Latin post coitum omne animal triste est (and variants): ‘after sexual intercourse every animal is sad’ (16th-cent. or earlier, perhaps originally in paraphrases of or commentaries on the pseudo-Aristotelian Problems). Compare pseudo-Aristotle Problems 955 a 22 καὶ μετὰ τὰ ἀϕροδίσια οἱ πλεῖστοι ἀθυμότεροι γίνοται ‘and after love-making most people become rather lacking in spirit’, rendered in a 1475 Latin translation as post coitum quoque venereum animo plerique omnes succumbunt, reddunturque tristiores. Compare Problemata Aristotelis ac philosophorum medicorum complurium 129 (in a section of problems ‘ex variis’, i.e. not from Aristotle) quare animal post coitum tristatur? respondetur, quia actus luxuriae est in se turpis et immundus, ‘why is an animal sad after sexual intercourse? The answer is, that an act of luxury is in itself foul and disgusting’.