Frank J. Wilstach, comp. A Dictionary of Similes. 1916.
Flags
There is affection in every employment, and it gives the spirit energy, and keeps the mind intent upon its work or study. This if it be not relaxed, becomes dull, and its earnestness flags,—as salt that has lost its savor, so that it has no pungency or relish; or as bended bow, which unless it be unbent, loses the power that it derives from its elasticity.
—Emanuel Swedenborg