scatterbrained

adjectives

lacking sense or discretion
"his rattlebrained crackpot ideas"; "how rattlepated I am! I've forgotten what I came for"- Glenway Westcott
rattlebrained, rattlepated, scatterbrained, scatty
guided by whim and fancy
"flighty young girls"
flighty, flyaway, head-in-the-clouds, scatterbrained