What does "Hasbunallahu wa ni'mal wakil" mean?

Word by word

• Hasbuna: "enough for us" or "sufficient for us".
• Allahu: Allah.
• Wa ni'ma: "and how excellent is".
• Al-wakil: the Trustee, the one you hand your affairs over to and rely on completely.

The singular form is "Hasbiyallahu" ("Allah is enough for me"), which you can use when praying alone about your own situation.

Where it comes from

After the Battle of Uhud, the believers were told that a large army had gathered against them and they should be afraid. Their response is recorded in the Quran:

Ibn 'Abbas said that Prophet Ibrahim said these words when he was thrown into the fire, and the Prophet Muhammad ﷺ said them when he was told that people had gathered against him (Sahih al-Bukhari 4563). Allah saved Ibrahim from the fire and returned the believers safely with His favour.

When to say it

• When you face injustice and can't fix it yourself, at work, in court, or in a family conflict.
• When anxiety about the future hits: money, health, immigration papers, a hard conversation. See What dua can I say when I feel anxious?
• When people mock or pressure you for becoming Muslim.
• When you've done everything you can and the rest is out of your hands.

Trust plus effort

This phrase isn't a way of saying "I'll do nothing". The companions who said it still prepared and went out. Tawakkul, trust in Allah, means doing your part and then letting go of the result. It turns worry into calm, because you know the outcome is in the best hands. See Does tawakkul mean I shouldn't worry?

Some people say it as a kind of complaint against a person ("Allah will deal with you"). It can be said when you're wronged, but the heart of the phrase is trust, not cursing. For a related phrase about relying on Allah's strength, see What does "La hawla wa la quwwata illa billah" mean?