Heptakis means seven times — the adverbial form of hepta (seven). It appears in Matthew 18:21-22 (Peter's question about forgiving seven times, and Jesus' response about seventy-seven or seventy times seven), Luke 17:4 (forgiving a brother seven times in a day), and in Revelation contexts. The number seven already implies completeness; seven times amplifies the sense of utter fullness.
When Jesus responds to Peter's proposed limit of forgiveness 'seven times' with 'seventy-seven times' (or 70x7), He is not providing a mathematical formula but destroying the concept of a forgiveness limit altogether. Heptakis in Peter's question reflects a generous standard — seven times already exceeds normal human patience. Jesus multiplies it to the point of absurdity: there is no ceiling on forgiveness because God has forgiven us an infinite debt (Matt 18:21-35, the parable of the unforgiving servant). The mathematics of grace always overflow every human calculation.