The best weapon a man has is that which protects him. خير سلاح المرء ما وقاه

Meaning, the best son and family a man has is that which provides him with what he needs.
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 The closest English equivalent to the proverb's meaning:

The best support is the one that sustains you.

Or, more accurately:

A good son is his father's best protection.

For the general meaning, which implies that the best family or helper is the one who meets your needs and protects you:

A friend in need is a friend indeed.

However, this latter meaning is broader and more common, and it doesn't focus on the concept of "sufficiency and protection" like the Arabic proverb.

It can also be expressed with a similar English proverb:

The best defense is a reliable ally.

That is: The best thing a person can rely on is the one who protects and sustains them.

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يعني، خير ولد الرجل وأهله ما كفاه ما يحتاج إليه.