My thought is that the "a-" prefix in this sense probably comes from the Germanic use "to show a state, condition, or manner"; http://en.wiktionary.org/wiki/a-#

My thought is that the "a-" prefix in this sense probably comes from the Germanic use "to show a state, condition, or manner"; http://en.wiktionary.org/wiki/a-#Etymology_2 http://en.wiktionary.org/wiki/a-#Etymology_2 . It's no longer used to make new words, but the "a-" form remains in many words, including abloom, aflame, and abuzz. The first known use of the word "astir" is from 1765, says Merriam-Webster at http://www.merriam-webster.com/dictionary/astir http://www.merriam-webster.com/dictionary/astir . This is what I would expect if "stir" was a word, "a-" was a possible mechanism to create new words, and eventually people started to use "astir. Etymonline gives a first known use as 1823: http://www.etymonline.com/index.php?allowed_in_frame=0&search=astir&searchmode=none http://www.etymonline.com/index.php?allowed_in_frame=0&searc... : > "up and about," 1823, from phrase on the stir, or from Scottish asteer; from stir. Old English had astyrian, which yielded Middle English ben astired "be stirred up, excited, aroused." The root "stir" has a longer heritage. Etymonline at http://www.etymonline.com/index.php?term=stir&allowed_in_frame=0 http://www.etymonline.com/index.php?term=stir&allowed_in_fra... says: > Old English styrian "to stir, move; rouse, agitate, incite, urge" (transitive and intransitive), from Proto-Germanic * sturjan (cognates: Middle Dutch stoeren, Dutch storen "to disturb," Old High German storan "to scatter, destroy," German stören "to disturb"), from PIE * (s)twer- (1) "to turn, whirl" (see storm (n.)). Hindi is another descendant of Proto Indo-European, so that may be where there's a connection to "asthir". However, do bear in mind that the surface similarity is false - astyrian would be linguistically closer to the Hindi than astir.