No Holds Barred (usually adj., often hyphenated)

What does the idiom “No Holds Barred (usually adj., often hyphenated)” mean?

Definition:

Unrestricted, without rules

For example:

“At a meeting today, Chad and Joe got into a no-holds-barred argument, but at the end they agreed to disagree.”

Notes:

The original sense of the phrase referred to wrestling.