expressions - A man who keeps his word

An old-fashioned expression goes,

He's a man whose word is his bond.

The word bond is borrowed from the world of finance and insurance, and it concerns the guarantee of payments in the future, given certain criteria and circumstances which are laid out in a contract, or bond.

A man or woman who gives you their word (or guarantee) they will do something in the future is a man or woman whose word alone, and not an official-looking contract on a piece of paper, is sufficient, to guarantee their performance.

In other words, a person whose word is his bond is a person with whom you can forge an agreement by simply shaking hands to "seal the deal."

In a litigious society such as ours in the US, shaking hands to seal a deal is rarely heard of, since a written contract is more persuasive in a court of law when and if the agreement is breached.