Word of the Day:

1 : to harden (a ferrous alloy) so that the surface layer is harder than the interior

2 : to make callous or insensible

All locks at the warehouse will be checked to ensure that they are case-hardened and in good working condition.

Although humans had learned how to case-harden wrought iron by about 1200 B.C.E., the word "case-harden" didn't forge its way into the English language until the late 17th century. The term comes from the hard surface layer, or case, that is created on some metals when they are exposed to carbon, ammonia, or other substances at very high temperatures. By the early 18th century, the term was being used figuratively. The participial "case-hardened" is also used as an adjective (as in "case-hardened steel" and "case-hardened hearts") and is now more common than the verb in both technical and nontechnical contexts.