Holy means cut off from ordinary use and set apart for God, not primarily morally pure.
The main Hebrew word is qadosh, with the noun qodesh. Its root sense is separation. A holy object, day, or person is one marked off from common use and reserved for God. The Sabbath is holy because it is cut out of the week. The temple vessels are holy because they may never be used for dinner at home. Moral purity often follows from this, since anything near God tends to need cleaning up, but separation comes first, not second.
The Greek word is hagios, used the same way across the New Testament: God is hagios, believers are called hagioi ("holy ones," often translated "saints"), and the Spirit is the Holy Spirit, the one set apart to indwell and consecrate. English holy has to carry both the Hebrew idea of set-apart-from and the built-up moral weight the word has gathered over centuries of church use, and it tends to lean hard toward the moral sense, quietly losing the separation.
“Do not come any closer,” God said. “Take off your sandals, for the place where you are standing is holy ground.”
The ground is holy not because it is morally clean but because God's presence has claimed it.
“Speak to the whole congregation of Israel and tell them: Be holy because I, the LORD your God, am holy.
God commands Israel to be holy as he is holy, tying human calling directly to God's own set-apart nature.
And they were calling out to one another: “Holy, holy, holy is the LORD of Hosts; all the earth is full of His glory.”
The seraphim's threefold cry names holiness as God's defining, unrepeatable attribute.
for it is written: “Be holy, because I am holy.”
Peter quotes Leviticus, carrying the same command about set-apart living straight into the Greek New Testament.
Most English Bibles render both qadosh and hagios simply as "holy," with hagioi usually becoming "saints" rather than "holy ones," which obscures the link between what God is and what his people are called. "Sanctify" and "consecrate" translate the related verb forms and keep the separation idea more visibly, but they sound formal and rarely make it into everyday reading, so most readers only ever meet the flattened adjective.
People assume holy means something like extremely good or morally spotless, so "holy" almost becomes a synonym for sinless. But a bowl, a mountain, or a nation can be called holy in the Bible with no moral content implied at all, only that it belongs exclusively to God. Being holy is first about whose you are, and only from there about how you live.
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