Middle English: reduced form of Old English alswā 'similarly' (see also)
For a discussion of whether it is correct to say he’s not as shy as I rather than he’s not as shy as me or I live in the same street as she rather than I live in the same street as her see personal pronoun (usage).
A word that can be used in three main ways:Subordinating conjunctionIt can introduce a number of different types of adverbial clause: TimeAs the train drew into Victoria station, Gloria softened. ReasonAs they are fast-drying ... the application technique is slightly different. MannerThe talk that night was about experiments carried out to explain why people behaved as they did. CommentThat, as I understand it, is the law. The commonest of these four uses are the first two; in them as can mean ‘while’ or ‘because’. Occasionally this can cause confusion if a sentence is carelessly constructed. For example:I left the farm as it was getting late. Does this mean when it was getting late, or because it was getting late?PrepositionShould I get a job as a barmaid? AdverbIt can also be used as an adverb in comparisons:He’s as happy as a sandboy.