twardnieć po angielsku:

People usualy respond to a tense situation by becoming tense themselves, their body stiffen.

The law would stiffen penalties for tax evasion.

Brother men who live after us, Harden not your hearts against us.

At night, I put my bell pepper plants at the open window, so they can harden off a bit before I plant them outside, cause now they still have such thin stems.