The word tetelestai was also written on business documents or receipts in New Testament times to indicate a bill had been paid in full. The Greek-English lexicon by Moulton and Milligan says this:
“Receipts are often introduced by the phrase sic tetelestai, usually written in an abbreviated manner...” (p. 630). The connection between receipts and what Christ accomplished would have been quite clear to John’s Greek-speaking readership; it would be unmistakable that Jesus Christ had died to pay for their sins.
dude 2: "it's already been paid in full by Jesus man... tetelestai!"