sklērunō

This verb means to make something hard, and in the New Testament it is only ever used of people. Nothing physical is hardened in these six verses: what stiffens is a person's response to God, until an appeal that ought to move them cannot get in at all. The picture behind the word is an ordinary one, something that has set and will no longer bend, and Greek medical writers used it that way for skin and tissue long before it described a will.

Three of the six occurrences are the same quoted line, "do not harden your hearts", which the letter to the Hebrews takes from Psalm 95 and repeats at Hebrews 3:8, Hebrews 3:15, and Hebrews 4:7. The repetition is the argument: hardening is something people do to themselves, and they do it in the present, by hearing God's voice today and not answering it.

The other three occurrences move the responsibility around, and the word by itself does not tell you where it lands.

• In Hebrews 3:13 a person may be hardened by the deceit of sin, with nobody named as the one doing the hardening.
• In Acts 19:9 some of Paul's hearers in Ephesus simply grew hard and would not believe what he was arguing.
• In Romans 9:18 God is the one who hardens whom he wills, on the pattern of Pharaoh in Exodus.

So a reader has to watch the whole sentence, not just the verb, to see whose act the hardening is.

Greek had a second verb for a closed heart, πωρόω, and it pictures something different: being dulled or blinded so that a person cannot take in what is in front of them. This word keeps the sense of something set rigid instead. The related adjective σκληρός shows the everyday feel of the family, used of a harsh master, a hard saying, and rough words, while the noun σκληροκαρδία names the settled condition that hardening leaves behind.