Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
brownnose (n., v.), brownnoser (n.)
The verb means to curry favor by obsequious behavior, and both nouns (the doer) and the verb are based on an extension of the Vulgar slang locution, “to kiss (someone’s) ass.” The terms were taboo until fairly recently in any mixed group, but like some other terms based on excretory acts and products they now seem to retain their figurative meanings while losing or at least suppressing their origins. All are slang in any use; choose your audience with care.
