Entry 1 sense Webster, 1913 featherbrained · feath·er·brained · IPA 01 a. Giddy; frivolous; lacking seriousness; given to frivolity; feather-headed. 1. Giddy; frivolous; lacking seriousness; given to frivolity; feather-headed.[Colloq.] Syn. airheaded, dizzy, empty-headed, giddy, lightheaded, light-headed, silly.