Is meep a real word?

"Meep" does not appear in a dictionary. However, it is a slang form of onamatopoeia (words that are spelled like sounds, for example, beep, boom, or baaa). According to Wikipedia, it is a form of "beep," so it imitates the sound of a car or bicycle horn. Still, it's in no formal dictionary, so I'd keep it out of your essays.