Mat 18:30 And he would not: but went and cast him into prison until he should pay the debt.

A debt is a bond, which is an obligation, a loss of freedom. An unresolved debt leads to a greater loss of freedom. For most of human history, this meant prison. Not paying a debt was exactly the same crime as stealing.

Christ uses financial transactions to illustrate social interactions among people because financial transactions are the purest form of such interaction. However, he was the first to suggest that giving people, even those indebted to you, greater freedom rather than less was best course. Christ was less concerned with deterence and more concerned with general productivity. Personally, I have never understood how people in debtor's prison were able to resolve their debts.

"Went" is from aperchomai, which means "go away" and "depart from." It can mean to depart from one place and arrive at another.

"Prison" is from phulakê, a form of phulax, which means a "watcher," "guard," "chain," "keeper," and "a place where people are watched." The phulakê form is found only in the NT where it is clearly used to mean prison.

"Pay" is from is from apodidômi, which means literally, "give back" or "give up," and also means "restore," "return," "yeild," "give account," and "sell."

"Debt" is from opheilô, which means "to owe," "to have to pay," and "to account for."