- 1.1In the process of (an activity)
- 1.2In a specified state.
Old English, unstressed form of on.
variant spelling of ad- assimilated before sc, sp, and st (as in ascend, aspire, and astringent)
Sense 1 is an unstressed form of of; sense 2 is from Anglo-Norman French a- (Old French e-, es-), from Latin ex.
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