Correct spelling, explanation: this word is the correct spelling, according to its origins, which are French (guarde). Guard is spelled with ua, not au, just like in the French original word. Gaurd is therefore an incorrect form.

Definition of guard:
1. noun, a person who protects a person or a thing from stealing, escaping, etc.
The security guard didn’t see anything suspicious about her.
2. verb, to protect from stealing, escaping, etc.
The dragon guards over the princess in her magic castle.

Idioms and phrases with guard:
→ to catch sb off guard, to come unexpected to someone:
I was caught off guard when you gave me these concert tickets!
→ to be on one’s guard, to be careful and prepared:
Always be on your guard when you go back home late at night.

Incorrect spelling, explanation: roots of the correct form guard are French and the first known appearance in English is from the 15th century. Due to the French original spelling, guarde, it is wrong to spell this word with vowels au. It should be spelled with ua, and thus the correct form is guard.

Incorrect spelling

Incorrect spelling