An ignoramus is a word for a person without any intelligence, an extremely dumb individual. It is a pejorative term meant to be an insult.
The plural is ignoramuses. Some dictionaries list ignorami as a variation of the plural, but this is a backformation by those who suppose since ignoramus comes from Latin that it would have the Latin –i plural. However, in the original Latin, ignoramus was a verb, not a noun, and would still have the -es plural.