When Jesus spoke these words from the cross he was using a simple everyday Greek word that all who heard it would have understood.
Slaves and servants used the word on completing tasks saying "Tetelestai"; it is completed as the master required. The priests would have understood it from when they inspected the sacrifices brought to the temple. The priest would inspect the animal offered and if it was spotless and without blemish he would declare "Tetelestai"; it is perfect. Merchants and traders would have understood Jesus. Upon completing a transaction the merchant would declare "Tetelestai"; it is paid in full.
Jesus on the cross declared "Tetelestai"; it is completed as the Father required, Jesus was the perfect sacrifice without spot or blemish and Jesus paid the price for our sin in full.
"It is finished" - Hallelujah!