Usage guides invariably include prescriptive rules codifying meanings for words like aggravate and infer. The guides do not always pronounce proscribed forms as incorrect, but when they claim incorrect meanings, irrespective of actual usage, they suggest the existence of some principle of language that can serve as an authority in correctness judgments about word meanings akin to grammatical "rules" for correctness judgments about morphology and syntax. What those principles might be is the topic of this paper. The most basic assumed principle is that words have discrete meanings and that speakers and writers use words incorrectly if they use them for senses they do not have. But this assumption does little more than restate the proscription. For general principles that can be used as an authority for claims of correctness, the most frequent appeal in usage guides is to etymology in both its narrow sense of word origins and its wider sense of word histories. Appeals to the related notions of "confusion," "differentiation," and "slipshod extension" support and reveal the assumptions about etymology underlying correctness claims.