level best

Noun

level best (plural level bests)

1. (idiomatic) The very best that one can do.
   • Then came a pause, each man aiming his level best, as indeed one is likely to do when one knows that life itself depends upon the shot.1885 September, H[enry] Rider Haggard, “Solomon’s Road”, in King Solomon’s Mines, London; Paris: Cassell & Company, […], published 1887, →OCLC, page 102
   • Val walked out behind his mother, chin squared, eyelids drooped, doing his level best to despise everybody.1920, John Galsworthy, “Dartie versus Dartie”, in In Chancery (The Forsyte Saga; 2), London: William Heinemann, →OCLC, part II, page 186