crack·brain 
(krăkbrān′)
(krăkbrān′)n.
A foolish or eccentric person.
crackbrained′ adj.
- involution
- involutional
- involve
- involved
- involvedly
- involvement
- involver
- invulnerability
- invulnerable
- invulnerableness
- invulnerably
- in waiting
- inward
- inward dive
- inwardly
- inwardness
- inweave
- in well with
- in wheelhouse
- in with
- in/within bounds
- in/with reference to
- in/with regards to
- in/with regard to
- inwrought