1. Lacking sense or discretion
   - rattlebrained, rattlepated, scatty [Brit] 
2. Guided by whim and fancy
   - flighty, flyaway, head-in-the-clouds

See also: foolish, frivolous

• fallaciously
• fallaciousness
• fallacy
• fallal
• fallback
• fallboard
• fallen
• fallen angel
• fallen arch
• faller
• fallibility
• fallible
• fallibly
• falling
• falling off
• falling out
• falling star
• falloff
• Fallopian tube
• Fallopio
• Fallopius
• Fallot
• Fallot's syndrome
• Fallot's tetralogy
• fallout