Standing with your legs on both sides of something, straddling it, like sitting on a horse or standing over something with one leg on each side.
From 'a-' (on, in) plus Old English 'stride' meaning 'to step or stand.' Literally means 'in the position of stride.'
Astride is a word that creates such a vivid image—it literally puts something between your legs, so you're powerful and controlling whatever you're straddling. That's why it's used metaphorically to describe a giant colossus or empire that dominates a place.
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