The phrase "to take someone down a peg" is historically related to ships. It originally comes from the practice of raising and lowering flags on sailing ships. Ships would raise their flags to indicate their status or to show respect. Flags were raised on pegs (or "halyards") at different heights on the mast, with higher positions symbolizing greater honor or authority.

When someone "takes a peg down," it means that the flag is lowered a bit, signifying a reduction in that honor or status. When used metaphorically, it means to humble someone who is acting too arrogant or self-important. Thus, taking someone "down a peg" suggests putting them in their place or reminding them that they are not as important as they think they are.

This nautical origin later evolved into the more general expression of humbling someone.