Capable of being accelerated or made to move faster.
From Latin accelerare (to hasten) + -able (capable of). The root comes from ad- (to) + celer (swift), ultimately from Proto-Indo-European *kel- meaning 'to drive or move quickly.'
The -able suffix is one of English's most productive word-formation tools, turning almost any verb into an adjective meaning 'capable of being X-ed'—which is why you can create grammatically sensible words like 'accelerable' even if you've never seen them before!
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