Other forms: straddles, straddling, straddled
To straddle is to sit or stand with a leg on either side. A bicycle, a fence, a horse.
The posture gives the word its meaning, and everything else follows from it. Geography borrows it for anything lying across a line, so a town can straddle a border and a river can straddle two counties. The figurative use is about refusing to choose, since straddling two positions means keeping a foot in each, and that is usually said with disapproval. Finance has a technical sense for a particular pair of options.
As a verb, attested since the 1560s. Most likely, an alteration of dialectal striddle. The noun is first attested in the 1610s.
Etymology from Wiktionary, CC BY-SA