Brashness comes from brash ('impudent, hasty') + the noun-forming suffix -ness. The adjective brash is of uncertain 19th-century origin. Morphologically, brashness is transparent: it draws on brash ('impudent, hasty') + the noun-forming suffix -ness. Used as a noun, brashness now means "the quality of being bold, hasty, or impudent", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in related forms such as brash and brashly. That uncertainty is part of the entry: the safest account is the attested form and the limited comparisons preserved with it.