featherbrainedfeath‧er‧brained /ˈfeðəbreɪnd $ -ər-/ adjective 
extremely silly
extremely silly- investigate
- investigation
- investigative
- investigator
- investigatory
- investiture
- investment
- investment bank
- investment banker
- investment banking
- investment club
- investor
- inveterate
- invidious
- invigilate
- invigilator
- invigorate
- invigorating
- invincibility
- invincible
- inviolability
- inviolable
- inviolate
- invisibility
- invisible