The term "unforgivable sin" refers to a grave offense against God, that is considered so terrible and heinous that it cannot be forgiven or pardoned. In Christian beliefs, this concept is primarily associated with blasphemy against the Holy Spirit - an act of willful rejection or defiance towards God's grace and mercy. The idea behind this notion is that certain sins are so egregious in nature that they cut a person off from God permanently, making it impossible for them to be forgiven or reconciled with the divine. However, different religions have varying interpretations of what constitutes as an unforgivable sin and its consequences.