What is an adjective for excuse?

There isn't a single perfect adjective for "excuse" because the word itself implies a reason given to justify something, often something considered wrong or inappropriate. However, depending on the context, you could use words like:

For a weak or insincere excuse:

* Flimsy

* Lame

* Pathetic

* Preposterous

* Unsubstantiated

* Unconvincing

* Bogus

* Fabricated

For an excuse that is acceptable or reasonable:

* Valid

* Legitimate

* Justifiable

* Acceptable

* Plausible

For an excuse that is used frequently:

* Chronic

* Habitual

* Repetitive

For an excuse that is made up:

* Fabricated

* Invented

* Concocted

The best adjective to use will depend on the specific situation and the nuance you want to convey.