It’s because the pronunciation is technically a ‘y’ sound, and even though y is sometimes considered a vowel, ‘an’ is not used in front of words that start with the ‘y’ sound.
Answer: Context Clues
Explanation: If you have an example sentence of "She's such an ignoramus, an absolute idiot." The word you recognize (i.a idiot) would help you understand that ignoramus generally means the same thing.
