1610s, "insufficient," from French incompétent, from Late Latin incompetentem (nominative incompetens) "insufficient," from in- "not" (see in- (1)) + Latin competentem (see competent). Sense of "lacking qualification or ability" first recorded 1630s. The noun meaning "incompetent person" is from 1866. Related: Incompetently.
"contemptible person," 1914 in U.S. underworld slang, originally "incompetent or worthless criminal," perhaps from a sense of "person in the lowest position" and thus from heel (n.1).
"incompetent or foolish but likeable person," by 1930 (according to Partridge). There was a character Little Mook in a series of children's stories c. 1900.
"unscrupulous lawyer," 1843, U.S. slang, probably altered from German Scheisser "incompetent worthless person," from Scheisse "shit" (n.), from Old High German skizzan "to defecate" (see shit (v.)).
also potzer, "an incompetent chess player," especially one who doesn't know he is, by 1948, of uncertain origin. OED points to German patzen "to bungle," but notes that, though the form looks Yiddish, there doesn't seem to be such a word in Yiddish.
early 14c., "trailing part of a robe, gown, etc.," from trail (v.). The meaning "track or smell left by a person or animal" is also from 1580s. Meaning "path or track worn in wilderness" is attested from 1807. Trail of Tears in reference to the U.S. government's brutally incompetent Cherokee removal of 1838-9 is attested by 1908.
1640s, "without inherent force, having no power to act or respond," from French inerte (16c.) or directly from Latin inertem (nominative iners) "unskilled, incompetent; inactive, helpless, weak, sluggish; worthless," used of stagnant fluids, uncultivated pastures, expressionless eyes. It is a compound of in- "without, not, opposite of" (see in- (1)) + ars (genitive artis) "skill" (see art (n.)). In chemistry, "having no active properties, neutral" (1800), specifically from 1885 of certain chemically inactive, colorless, odorless gases. Of persons or creatures, "indisposed or unable to move or act," from 1774.
"overacting inferior performer," 1882, American English, apparently a shortening of hamfatter (1880) "actor of low grade," which is said (at least since 1889) to be from the old minstrel show song, "The Ham-fat Man" (attested by 1856). The song, a comical black-face number, has nothing to do with acting, but the connection might be with the quality of acting in minstrel shows, where the song was popular (compare the definition of hambone in the 1942 "American Thesaurus of Slang," "unconvincing blackface dialectician"). Its most popular aspect was the chorus and the performance of the line "Hoochee, kouchee, kouchee, says the ham fat man."
Ham also had a sports slang sense of "incompetent pugilist" (1888), perhaps from the notion in ham-fisted. The notion of "amateurish" led to the sense of "amateur radio operator" (1919).