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Definition of au grand sérieux in English:

au grand sérieux

Pronunciation /əʊ ˌɡrɒ̃ ˌsɛrˈjəː/

adverb

(also au grand serieux)

  • Very seriously, in all seriousness. Chiefly in "to take (also treat) au grand sérieux".

Origin

Mid 19th century; earliest use found in William Thackeray (1811–1863), novelist. From French au grand sérieux from au to the + grand + sérieux, use as noun of sérieux.