flightiest
fligh-ti-est
adjective
Most prone to being unreliable, changeable, or frivolous in behaviour or ideas.
"Of all her friends, she was the flightiest, never sticking to plans."
Thesaurus
Synonyms: skittish, flighty, spooky, excitable, nervous, flyaway, scatterbrained, frivolous, head-in-the-clouds