Anointing is generally reserved for sacred objects or sacred heads. Saints are anointed, I think. I imagine it as a kind of blessing, but with oil.
But Laertes is going to anoint his sword with poison. He’s trying to get around the whole murdering thing by covering it in trappings of religion.
This seems to be one of the finest tricks of a religion to proclaim clearly and distinctly “Thou shalt Not Kill” but put a little asterisk next to it that leads to a footnote of exceptions.
*Except in cases of revenge, or holy purpose, or if your sword is anointed or it’s your dad that got killed. In that case – go for it.