Many languages, especially in antiquity, have colloquial ways of phrasing “forever” or “a long time” with a superficially-specific count. In Japanese, “ten thousand years” can be used to indicate an indefinitely long period; in Ancient Hebrew, “40 days and 40 nights” does that job.
Given the number of such numerically-precise-but-pragmatically-vague sayings in many languages, and the apparent failure of them to converge beyond shared cultural contact (Classical Arabic has the same use pattern for “40”, as do many Middle Eastern languages from antiquity, though I’ll admit that my linguistic knowledge doesn’t do more than touch on this region superficially, other’n a few years of Modern Hebrew), I don’t think “arbitrary” quite captures it—they simply adopted a use pattern that was widespread in the time and place where they were.
Many languages, especially in antiquity, have colloquial ways of phrasing “forever” or “a long time” with a superficially-specific count. In Japanese, “ten thousand years” can be used to indicate an indefinitely long period; in Ancient Hebrew, “40 days and 40 nights” does that job.
But is there any known reason for picking 40 specifically? I wouldn’t expect the Jews to choose their numbers arbitrarily.
Given the number of such numerically-precise-but-pragmatically-vague sayings in many languages, and the apparent failure of them to converge beyond shared cultural contact (Classical Arabic has the same use pattern for “40”, as do many Middle Eastern languages from antiquity, though I’ll admit that my linguistic knowledge doesn’t do more than touch on this region superficially, other’n a few years of Modern Hebrew), I don’t think “arbitrary” quite captures it—they simply adopted a use pattern that was widespread in the time and place where they were.