The Spanish word 'mamerto', meaning 'foolish person', comes from the Latin personal name 'Mamertius'. This is an interesting case of a personal name evolving into a pejorative term, similar to how some English names like 'dumb john' or 'simple simon' became associated with certain characteristics. The name Mamertius likely became associated with foolishness or simple-mindedness in Spanish-speaking regions, eventually becoming a general term for a foolish person.

There don't appear to be any commonly used, simpler Spanish words that are etymologically related to 'mamerto', as it derives from a specific Latin personal name.

There aren't any common English words directly related to 'mamerto'. While English has borrowed many words from Latin, this particular Latin personal name 'Mamertius' didn't develop into any commonly used English terms.