What is the origin of the phrase 'Drastic times call for drastic measures'?
The origin of this proverb:
This is a variant of the proverb "Desperate diseases must have desperate remedies." This goes back to the Latin _extremis malis extrema remedia_ 'extreme remedies for extreme ills.'
The earliest English version given by the Concise Oxford Dictionary of Proverbs is the following:
1539 R. Taverner tr. _Erasmus' Adages_ 4 A stronge disease requyreth a stronge medicine.
(Thanks to Fred Shapiro, Yale Law Library)
My mother used to say something similar - "Needs must when the devil rides." This was in the context of having to take drastic measures when bad things happen. This version of the phrase is mentioned in the Wordsworth dictionary of idioms. Your version has the "when" at the beginning, and changes the meaning considerably - it becomes a warning that taking drastic measures is inviting bad things to happen. I haven't come across this…
'Coup' is a French origin loan word into English, as I'd say you are aware since you classified the question in 'French to English'. While the word 'coup' in the phrase 'counting coup' is still the same loan word from French as is used in 'coup d'etat', for example, the phrase 'counting coup' is of English origin.