Origin and history of sleuth

late 14c., "hollow at the base of the throat above the breastbone," from Old French esclot "hoofprint of a deer or horse," a word of uncertain origin, perhaps from Old Norse sloð "trail" (see sleuth). The anatomical sense is found in romances and in reference to wounds or blows; it is now rare or obsolete. The sense development is obscure, perhaps "from the resemblance of the jugular notch of the sternum to a hoof print" [Middle English Compendium]

The meaning "narrow opening into which something else can be fitted" is by 1520s. The sense of "opening in a machine for a coin to be inserted" is from 1888 (slot machine, one operated by inserting a coin in a slot, is attested by 1891).

The sense of "position in a list, place in a timetable" is attested from 1942. The meaning "middle of the (semi-circular) copy desk at a newspaper," the spot occupied by the chief sub-editor, is recorded from 1917. Slot car is attested by 1966, in reference to the slot in which the miniature car runs.