Origin and history of Sherlock

"tress of hair," Old English locc "lock of hair, curl" (plural loccas), from Proto-Germanic *lukkoz (source also of Old Norse lokkr, Old Saxon, Old Frisian, Dutch lok, Old High German loc, German Locke "lock of hair"), a word of uncertain origin. According to OED, perhaps from a PIE *lugnos- and related to Greek lygos "pliant twig, withe," Lithuanian lugnas "flexible" (see reluctance).

c. 1200, "exempt, free from guilt" (as in Sheer Thursday, the Thursday of Holy Week, the day before the Crucifixion); later schiere "thin, sparse" (c. 1400), a variant of skere, from late Old English scir "bright, clear, gleaming; translucent; pure, unmixed." The Middle English word might also be from or influenced by the Old Norse cognate scær "bright, clean, pure." Both of these are from Proto-Germanic *skeran (source also of Old Saxon skiri, Old Frisian skire, German schier, Gothic skeirs "clean, pure"), from PIE root *sker- (1) "to cut."

The sense of "absolute, utter" (sheer nonsense) developed by 1580s, probably from the notion of "unmixed, uncombined with anything else;" that of "very steep, straight up and down" (a sheer cliff) is recorded from 1800, probably from notion of "continued without halting." Especially of textile fabrics, "diaphanous, very thin and delicate," from 1560s. As an adverb from c. 1600; also sheerly.

late 12c., sloth, "track or trail of a person or beast," from Old Norse sloð "trail or track," as of a person in snow, a word of uncertain origin.

The meaning "detective" (1872) likely is ultimately a shortening of sleuth-hound "keen investigator" (by 1846). This is a figurative use of that word, which is attested from late 14c. in its original sense of "bloodhound," noted early 19c. as a Scotticism. The extension to police detectives would be from the notion of a relentless pursuer.

Sleuth alone as a name for "a detective" is attested from 1872 in sensational New York magazine stories featuring or credited to "Old Sleuth" the detective. (Sleuth also was the pseudonym of a Brooklyn newspaper correspondent active earlier in 1872.)

A "Richard Sleuth" is the main character in a sensational novel serialized in England in 1865 and published in 1866 as "Bound to the Wheel," by John Saunders. Richard Sleuth is not a detective but a relentless immoral schemer "physically a coward but intellectually cool, who pursues base ends by base means" according to a contemporary reviewer. Perhaps the character-name is based on sleuth-hound. The U.S. journalistic use six years later might be aware of Saunders's novel.

The series of U.S. crime-thriller stories and cheap novels continued popular through the 1880s and '90s, and the use of sleuth for “detective” in titles was a subject of a lawsuit among publishers. Compare hawkshaw, Sherlock, shamus.