"path for pedestrians on the side of a street," 1721, from side (adj.) + walk (n.). The use of sidewalk for pavement as one of the characteristic differences between American and British English has been noted at least since 1902.
c. 1200, "a tossing, rolling;" mid-13c., "an act of walking, a going on foot;" late 14c., "a stroll," also "a path, a walkway;" from walk (v.). The meaning "broad path in a garden" is from 1530s. Meaning "particular manner of walking" is from 1650s. Meaning "manner of action, way of living" is from 1580s; hence walk of life (1733). Meaning "range or sphere of activity" is from 1759. Sports sense of "base on balls" is recorded from 1905; to win in a walk (1854) is from horse racing (see walk-over). As a type of sponsored group trek as a fund-raising event, by 1971 (walk-a-thon is from 1963).
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Definitions of sidewalk from WordNet
walk consisting of a paved area for pedestrians; usually beside a street or roadway;