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

• isogonic line
• isogram
• isohel
• isohyet
• isolable
• isolate
• isolated
• isolating
• isolation
• isolationism
• isolationist
• isolationistic
• isolator
• Isolde
• Isole Egadi
• isoleucine
• isoline
• isomer
• isomerase
• isomeric
• isomerisation
• isomerise
• isomerism
• isomerization
• isomerize