"one who splits logs in rails for making a rail fence," 1853, from rail (n.1) + agent noun from split (v.). Usually with reference to or suggestion of U.S. president Abraham Lincoln, as it figured in his political biography.
"horizontal bar passing from one post or support to another," c. 1300, from Old French raille, reille "bolt, bar," from Vulgar Latin *regla, from Latin regula "rule, straight piece of wood," diminutive form related to regere "to straighten, guide" (from PIE root *reg- "move in a straight line").
In U.S. use, "A piece of timber, cleft, hewed, or sawed, inserted in upright posts for fencing" [Webster, 1830]. Used figuratively for thinness from 1872. By 1830s as "iron or steel bar or beam used on a railroad to support and guide the wheels." To be off the rails "out of the normal or proper condition" in a figurative sense is from 1848, an image from railroads.
1580s (transitive and intransitive), not found in Middle English, probably from a Low German source such as Middle Dutch splitten, from Proto-Germanic *spleitanan (source also of Danish and Frisian splitte, Old Frisian splita, German spleißen "to split"), from PIE *(s)plei- "to split, splice" (see flint).
U.S. slang meaning "leave, depart" first recorded 1954. Of couples, "to separate, to divorce" from 1942. To split the difference is suggested from 1715; to split (one's) ticket in the U.S. political sense is attested from 1842. To split hairs "make too-nice distinctions" is from 1670s (split a hair). Splitting image "exact likeness" is from 1880. To split the atom is from 1909.
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Definitions of rail-splitter
a laborer who splits logs to build split-rail fences;