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

• supremo
• supt.
• suq
• sur-
• sura
• surah
• sural
• surbase
• surcease
• surcharge
• surcingle
• surcoat
• surd
• sure
• sure as shooting
• sure thing
• sure-enough
• sure-fire
• sure-handed
• surefooted
• surely
• sureness
• surety
• surety bond
• suretyship