I was told in simpler terms its "knowing it's "the truth™", but willingly going out of your way to fight against it".
Interestingly, only the 144,000 actually are sealed with the holy spirit, so us mere earth dwellers cant possibly grieve it in the first place!
Watchtower May 15th, 2010
What Paul Meant
3 First of all, note Paul’s words recorded at Ephesians 4:30. He wrote: “Do not be grieving God’s holy spirit, with which you have been sealed for a day of releasing by ransom.” Paul did not want his beloved fellow believers to endanger their spirituality. It was with Jehovah’s spirit that they had been “sealed for a day of releasing by ransom.” God’s holy spirit was and still is a seal, or an advance “token of what is to come” for anointed integrity keepers. (2 Cor. 1:22) The seal signifies that they are God’s possession and are in line for heavenly life. Those finally sealed number 144,000.—Rev. 7:2-4.
4 Grieving the spirit can be the first step leading to the total loss of the influence of God’s active force in a Christian’s life. That this is possible is evident from what David said after he had sinned in connection with Bath-sheba. David repentantly begged Jehovah: “Do not throw me away from before your face; and your holy spirit O do not take away from me.” (Ps. 51:11) Only those anointed ones who prove “faithful even to death” will receive the “crown” of immortal life in heaven. (Rev. 2:10; 1 Cor. 15:53) Christians with an earthly hope also need holy spirit if they are to maintain integrity to God and receive his gift of life on the basis of faith in Christ’s ransom sacrifice. (John 3:36; Rom. 5:8; 6:23) Hence, all of us must guard against grieving Jehovah’s holy spirit.
How Could a Christian Grieve the Spirit?
5 As dedicated Christians, we can avoid grieving the spirit. This is possible if we ‘keep on walking and living by spirit,’ for then we will not be overcome by wrong fleshly desires and will not manifest ungodly traits. (Gal. 5:16, 25, 26) But that could change. We could grieve God’s spirit to a degree by allowing ourselves to drift slowly, perhaps almost imperceptibly, toward conduct condemned in God’s spirit-inspired Word.
6 If we continually went contrary to the direction of holy spirit, we would be grieving it and Jehovah, the Source of that spirit. An examination of Ephesians 4:25-32 will show us how we should conduct ourselves, and it can help us to avoid grieving God’s spirit.