What does "all for naught" mean?

All for naught is an idiom that means "all for nothing", e.g. Kate didn't get the promotion, and she felt her hard work was all for naught.

People often confuse all for naught with homophones for naught. Sometimes, people write "all for not" because "not" is similar to "nothing". Other times, writers misspell "naught" as "nought". This spelling, while used occasionally in archaic English, is no longer considered correct, and is sometimes used in British English to mean "zero". "Knot" is another homophone for "naught", but it makes no sense in the phrase all for naught.

Incorrect: I planned a huge, surprise engagement, but she broke up with me before I could propose. My plans were all for not.

Correct: I planned a huge, surprise engagement, but she broke up with me before I could propose. My plans were all for naught.

Incorrect: Don’t give up or your hard work will be all for nought.

Correct: Don’t give up or your hard work will be all for naught.