late Old English, sett, "appointed or prescribed beforehand;" hence "fixed, immovable, definite;" c.1300, of a task, etc., "imposed, prescribed;" past participle of setten "to set" (see set (v.)). By early 14c. as "ready." By 14c. with adverbs, "having a (specified) position, disposition, etc.;" by late 14c. as "placed, positioned;" to be set "be ready"
By 1510s as "formal, regular, in due form, deliberate;" 1530s as "placed in a setting, mounted." By c. 1600, of phrases, expression, etc., "composed, not spontaneous" (hence set speech, one planned carefully beforehand). By 1810 of the teeth, "clenched." The meaning "ready, prepared" is recorded from 1844.
By 1844 in reference to athletes poised to start a race, etc., or their muscles, "have or assume a rigid attitude or state." The exact phrase Get set! in the procedure of sprinting (after on your marks) is attested by 1890. A set piece, in theater, is "piece of free-standing scenery only moderately high, representing a single feature (such as a tree) and permitting more distant pieces to be seen over it" (by 1859); also, in the arts, "a painted or sculptured group" (1846).