1580s, in rhetoric, "professed doubt as to where to begin," from Latin, from Greek aporia "difficulty, perplexity, want of means, poverty," abstract noun from aporos "impassable, impracticable, very difficult; hard to deal with; at a loss," from a- "not, without" (see a- (3)) + poros "passage" (from PIE root *per- (2) "to lead, pass over"). Meaning "equality of reasons for or against" is by 1893.
An extensive bridge consisting, strictly of a series of arches of masonry, erected for the purpose of conducting a road or a railway a valley or a district of low level, or over existing channels of communication, where an embankment would be impracticable or inexpedient; more widely, any elevated roadway which artificial constructions of timber, iron, bricks, or stonework are established. [Century Dictionary]
But the word apparently was coined by English landscape gardener Humphry Repton (1752-1818) for an architectural feature, "a form of bridge adapted to the purposes of passing over, which may unite strength with grace, or use with beauty ...."