What is blasphemy, exactly?

One thing I get tired of is the culture blaspheming the Lord.

Psalms 74:10 O God, how long shall the adversary reproach? shall the enemy blaspheme thy name for ever?

Deuteronomy 16:21 - "Do not set up any wooden Asherah pole beside the altar you build to the LORD your God,"

Barnes' notes on blasphemy:

The word "blaspheme" originally means to speak evil of anyone; to injure by words; to blame unjustly. When applied to God, it means to speak of him unjustly; to ascribe to him acts and attributes which he does not possess; or to speak impiously or profanely. It also means to say or do anything by which his name or honor is insulted, or which conveys an "impression" unfavourable to God.

Eaton's Bible Dictionary: sacrilege-

The sin or crime of violating or profaning sacred things; the alienating to laymen, or to common purposes, what has been appropriated or consecrated to religious persons or uses.

In 2 Corinthians 6:16 Paul was urging the new believers not to join in with the wicked and the profane, not to attend the festivals where vestiges of the old false gods and the vibrancy of the new false gods still were worshiped openly. He said not to join in any way with their sacrilegious activities. Believers must separate from these activities, soundly and firmly, visibly and demonstrably. Paul wrote: "What agreement is there between the temple of God and idols? For we are the temple of the living God. As God has said: "I will live with them and walk among them, and I will be their God, and they will be my people."

John MacArthur:

The issue here is an issue of sacrilege. All false religion is demon worship. Listen, now remember an idol is nothing. You can carve an idol out of wood, you can make an idol out of stone, you can make an idol out of silver, you can make one out of gold. You can do whatever you want to paint one on a wall. You can form one out of marble, whatever it is. When you're done with it, it's nothing. But the religion and the ideology that it stands for is the teaching of demons. It is lies from the pits. It is the doctrines of demons coming from seducing spirits. So that what happens is demons impersonate the idol and you worship a demon in the idol, though you don't know it. It is a demon who creates the religion, who conducts the relationship with the worshiper. It is demon communion.

MacArthur was speaking of Manasseh in 2 Kings 21:1-4-

Manasseh was twelve years old when he became king, and he reigned in Jerusalem fifty-five years. His mother’s name was Hephzibah. He did evil in the eyes of the Lord, following the detestable practices of the nations the Lord had driven out before the Israelites. He rebuilt the high places his father Hezekiah had destroyed; he also erected altars to Baal and made an Asherah pole, as Ahab king of Israel had done. He bowed down to all the starry hosts and worshiped them. He built altars in the temple of the Lord, of which the Lord had said, “In Jerusalem I will put my Name.”

He put the idols back into the Temple where the name of the ONE HOLY God's name resided! One of the false gods they worshiped idolatrously was Asherah. Those old gods are NOT GONE. Satan still perpetuates them, or recycles them. In Josiah's day it was Asherah. In Paul's day it was Diana. In our day, Molech has become "Abortion." The queen of heaven mentioned in Jeremiah 7:18 and  Jeremiah 44:17-25 is now Mary, Co-redemptrix of the Catholics.

GotQuestions explains that blasphemy is:

to speak with contempt about God or to be defiantly irreverent. Blasphemy is verbal or written reproach of God's name, character, work, or attributes.
Blasphemy was a serious crime in the law God gave to Moses. The Israelites were to worship and obey God. In Leviticus 24:10–16, a man blasphemed the name of God. To the Hebrews, a name wasn’t just a convenient label. It was a symbolic representation of a person’s character. The man in Leviticus who blasphemed God’s name was stoned to death.

The truth is, every time we misrepresent God in word or in behavior, we blaspheme. It's not just actively worshiping a stone false god that enacts a blasphemy but our own misrepresentation of Him. He is holy and perfect. We should be careful, oh so careful, to ensure that our lives and our words represent Him rightly as the God He is.

Who is like You among the gods, O LORD? Who is like You, majestic in holiness, Awesome in praises, working wonders? (Exodus 15:11)