легкомысленный

1) General subject: air-headed, airy, careless, casual, cavalier, daft, empty pated, empty-headed, empty-pated, fast, flip, flippant, fly away, fly-away, frivolous, giddy, giddy brained, giddy headed, giddy-brained, giggish, gossipy, hare-brained, harebrained, harum scarum, harum-scarum, high-hearted, highty tighty, hoity toity, hoity-toity, jejune, light, light minded, light-headed, light-minded, lightheaded, lightsome, puerile, scatter-brained, shallow hearted, shallow-hearted, shatter-brained, thoughtless, unreflecting, unthinking, carefree, play fast and loose, giddy-pated, irresponsible, silly, mindless, helter skelter

2) Colloquial: corky, scatterbrained, ditsy

3) Rare: volage

4) Scottish language: hellicat

5) Jargon: bliss ninny (Tiffany is such a bliss ninny! Тиффани такая лекгомысленная!), breezy (Your breezy attitude burns me out. Твоё легкомысленное отношение истощает меня.), in a twitter, dingy

6) leg.N.P. lighthearted, lightmindedness, trifling

7) Makarov: empty (о человеке), frivolous (о человеке), giddy-headed

8) Phraseological unit: brain-dead