One scholar suggests that the old familiar
saying may have originated with the similar-sounding Dutch expression
Er lif t"el baerd, which means "I should betray another." More likely
the idea behind the phrase is in the noiseless flight of a bird,
reinforced by a biblical passage from Ecclesiastes 10:20: "Curse not the
king, no not in the thought . . . for a bird of the air shall carry the
voice, and that which hath wings shall tell the matter." Used by
Shakespeare and Swift, the expression dates back to at least the sixteenth
century.