steel one's heart against

steel one's heart against

Also, harden one's heart. Suppress one's feelings for, as in He finally steeled his heart against them and refused the loan, or You'll just have to harden your heart and tell them the truth. This metaphoric idiom transfers making something hard to rendering oneself insensible or unfeeling. Versions of it date from the late 1500s. Also see heart of stone.
See also: heart, steel