E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898. defines 'pis-aller' as 'dernier ressort'. English Wiktionary translates the etymons.
But neither expounds the etymology. I can understand how "pis ressort", if this exists, can signify 'dernier ressort'. But I'm befuddled because "aller" doesn't signify "ressort".