Au Grand Sérieux
(French) In sober earnest.
“We are not asked to take these narratives au grand sérieux. They are rather sketches of the past, illustrating what could have been done, and may be done again by women. ...” —Notes and Queries (Notes on Books, June 10, 1893, p. 459).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894