To be greater in number than; to exceed in quantity or amount.
English prefix 'out-' (to exceed or surpass) combined with 'number.' The prefix 'out-' originally meant beyond or outside, and came to mean 'to do better or more than' another party.
The prefix 'out-' is a verb modifier that essentially means 'to do better at'—so you 'outrun' someone by running faster, 'outthink' someone by thinking better, and 'outnumber' them by having more bodies in the room.
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