Is it ensure, insure, or assure?
also scatter-brain, "thoughtless, giddy person, one incapable of serious, connected thought," 1764 (scatter-brained), from scatter (v.) + brain (n.). Related: Scatterbrained. Cf. scatter-good "spendthrift."
A silly or stupid person, esp one who cannot attend properly to a subject or get simple things done; ditz, rattlebrain: He's such a scatterbrain I don't look for a plausible theory from him (1790+)