Here's why:
* Noun: It refers to a reason given to justify a fault or mistake.
* Example: "He gave a flimsy excuse for being late."
* Verb: It can also mean to try to justify a fault or mistake.
* Example: "He tried to excuse his bad behavior."
Adjectives describe nouns. You might use an adjective to describe an excuse, such as "a lame excuse" or a "believable excuse".