He does not guard himself well who is not always on his guard.

Understand the thought

Meaning in plain English

Protection depends partly on continuing to notice changing conditions. A precaution taken once may become ineffective if its assumptions or surroundings later change.

What the words are saying

The repeated guard image stresses attention that continues beyond the first protective action. A locked door, a checked plan or a clear agreement can still need reconsideration when circumstances change. The saying’s demand to be always on guard is too absolute for ordinary life. A workable interpretation uses periodic checks, shared responsibility and proportionate safeguards instead of permanent suspicion. Ask what could change, how that change would be noticed and who would respond. Good protection should make daily activity more secure without consuming all the attention needed to live it.

Everyday example

A team reviews who has access to shared records when roles change, rather than assuming last year’s permissions remain appropriate.

When to use it

Use it to discuss maintenance of an existing precaution.

Agent-assisted editorial interpretation. This explains the words; it does not by itself verify their author or earliest origin.