To put something back together again after it has been taken apart or scattered.
From 're-' (again) plus 'assemble,' which comes from Old French 'assembler' (to gather together), from Latin 'ad-' (to) plus 'simul' (together). The prefix 're-' became common in English from Latin-influenced word formation.
The root of 'assemble' literally means 'put together at the same time'—so reassemble is putting back together the things that go together! It's the same root that gives us 'simultaneous.'
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