Hebrews 10:26

"explain away this verse"?
What is sin?

Why not let the very passage explain it?

26If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, 27but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. 28Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. 29How much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace?

There is the wilful sin. The wilful sin is doing those things.

Since it is the book of Hebrews, it was written to Hebrews...
It is about those Hebrews who saw Christ's ministry and instead of believing in Him for eternal life, they rejected Him. It is usually the case when someone sins that they know it is wrong, and it is willful.

It is saying, for those who know about Christ, the old way of getting your sins forgiven, by the sacrifices offered by the High Priest, etc. won't work anymore for them. The forgiveness of sins is only through Christ now. If they reject Him and think the old ways can save them, they are wrong.