/kays"hahr'dn/ , v.t.
1. Metall. to make the outside surface of (an alloy having an iron base) hard by carburizing and heat treatment, leaving the interior tough and ductile.
2. to harden in spirit so as to render insensible or insensitive to external impressions or influences; make callous.
[ 1670-80; CASE 2 + HARDEN ]