Step 1: Understand the word.
Impeccable means without fault or error - perfect, flawless, precise.
Step 2: Identify the antonym.
Its opposite should mean having faults or errors.
Step 3: Eliminate and confirm.
Flawless, perfect, and precise are all synonyms of impeccable, not antonyms. Defective means having flaws or faults, which is the direct opposite of impeccable.
\[ \boxed{\text{defective}} \]