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

• AstroTurf
• astucious
• astute
• astutely
• astuteness
• astylar
• Asuncion
• asunder
• Asur
• Asura
• Asurbanipal
• Asvina
• Asvins
• Aswan
• Aswan High Dam
• aswoon
• asylum
• asylum seeker
• asymmetric
• asymmetrical
• asymmetrically
• asymmetry
• asymptomatic
• asymptote
• asymptotic