Other forms: hardened, hardening, hardens
To harden is to make or become hard. Harden the steel, the glue hardened, harden your position.
In metalwork it is a controlled process — heating and quenching, then tempering to take the brittleness back out. Of glue, concrete and resin it means setting. Figuratively it means becoming less yielding: harden your heart, positions hardened, which is standard in reporting negotiations. Note hardened, meaning experienced beyond feeling; and that harden is hard plus -en.
1verbhardenTo make or become hard.
“Leave the melted chocolate in the fridge until it hardens.”
2verbyieldingTo become less yielding.
“After the attack, public opinion hardened against the rebels.”
harden is a making hard.
Middle English hardenen is hard plus -en, the ending that makes verbs meaning to make more so. Sweeten, soften and deepen are built the same way.
Old English
hierdan
Middle English
hardenen
English, today
harden
From Middle English hardenen, from und + harden, from Old English hierdan. From Middle English hardenen, equivalent to hard + en. Cognate with Danish hærdne (“to harden; cure”), Swedish hårdna (“to harden”), Norwegian herdne (“to harden”), Icelandic harðna (“to harden”).
Etymology from Wiktionary, CC BY-SA