"blooming, being in flower; apt to effloresce," 1741, from Latin efflorescentem (nominative efflorescens), present participle of efflorescere "to bloom, flourish," from assimilated form of ex "out" (see ex-) + florescere "to blossom," from flos "flower" (from PIE root *bhel- (3) "to thrive, bloom").
late 15c., "laborious attempt, strenuous exertion," from French effort, from Old French esforz "force, impetuosity, strength, power," verbal noun from esforcier "force out, exert oneself," from Vulgar Latin *exfortiare "to show strength" (source of Italian sforza), from Latin ex "out" (see ex-) + Latin fortis "strong" (see fort).
Effort is only effort when it begins to hurt. [José Ortega y Gasset, writing of Goethe in Partisan Review, vol. xvi, part ii, 1949]
Related: Efforts "voluntary exertion," also "result of exertion."
Latin frontus had a sense of "ability to blush," but the literal sense of effrontery often has been taken to be "putting forth the forehead." Forehead in Johnson's Dictionary (1755) has a secondary sense of "impudence; confidence; assurance; audaciousness; audacity." English had an earlier verb effront "treat with effrontery" (17c.).