blackleg n.

Old English blæc "absolutely dark, absorbing all light, of the color of soot or coal," from Proto-Germanic *blakaz "burned" (source also of Old Norse blakkr "dark," Old High German blah "black," Swedish bläck "ink," Dutch blaken "to burn"), from PIE *bhleg- "to burn, gleam, shine, flash" (source also of Greek phlegein "to burn, scorch," Latin flagrare "to blaze, glow, burn"), from root *bhel- (1) "to shine, flash, burn." The usual Old English word for "black" was sweart (see swart).

The same root produced Middle English blake "pale," from Old English blac "bright, shining, glittering, pale;" the connecting notions being, perhaps, "fire" (bright) and "burned" (dark), or perhaps "absence of color." According to OED, in Middle English "it is often doubtful whether blac, blak, blake, means 'black, dark,' or 'pale, colourless, wan, livid' "; and the surname Blake can mean either "one of pale complexion" or "one of dark complexion."  

Black was used of dark-skinned people in Old English. Of coffee with nothing added, attested by 1796. The meaning "fierce, terrible, wicked" is from late 14c. The figurative senses often come from the notion of "without light," moral or spiritual. Latin niger had many of the same figurative senses ("gloomy; unlucky; bad, wicked, malicious"). The metaphoric use of the Greek word, melas, however, tended to reflect the notion of "shrouded in darkness, overcast." In English it has been the color of sin and sorrow at least since c. 1300; the sense of "with dark purposes, malignant" emerged 1580s (in black art "necromancy;" it is also the sense in black magic). 

Black drop (1823) was a liquid preparation of opium, used medicinally. Black-fly (c. 1600) was a name given to various insects, especially an annoying pest of the northern American woods. Black Prince as a nickname of the eldest son of Edward III is attested by 1560s; the exact signification is uncertain. Black flag, flown (especially by pirates) as a signal of no mercy, is from 1590s. Black dog "melancholy" is attested from 1826.

Black belt is from 1870 in reference to district extending across the U.S. South with heaviest African population (also sometimes in reference to the fertility of the soil); it is attested from 1913 in the judo sense, worn by one who has attained a certain high degree of proficiency. Black power is from 1966, associated with Stokely Carmichael. Black English "English as spoken by African-Americans," is by 1969. The Black Panther (1965) movement was an outgrowth of Student Nonviolent Co-ordinating Committee. Black studies is attested from 1968.

late 13c., from a Scandinavian source, probably Old Norse leggr "a leg, bone of the arm or leg," from Proto-Germanic *lagjaz (cognates Danish læg, Swedish läg "the calf of the leg"), a word with no certain ulterior connections. Perhaps from a PIE root meaning "to bend" [Buck]. For Old Norse senses, compare Bein, the German word for "leg," in Old High German "bone, leg" (see bone (n.)). Replaced Old English shank (n.), itself also perhaps from a root meaning "crooked."

Distinguished from an arm, leg, or fin in being used for support. Of triangle sides from 1650s (translating Greek skelos, literally "leg"). Extended to furniture supports from 1670s. Meaning "part of pants which cover the leg" is from 1570s. By 1870s as an adjective it had a salacious suggestion of artistic displays focused on the female form, such as leg-piece in theater jargon, leg-business as slang for "ballet."

The meaning "a part or stage of a journey or race" (1920) is from earlier sailing sense of "a run made by a ship on a single tack when beating to windward" (1867), which was usually qualified as long leg, short leg, etc. Slang phrase shake a leg is attested from 1869 as "dance," 1880 as "hurry up." To be on (one's) last legs "at the end of one's life" is from 1590s, the notion is of something that serves one for support and keeps one moving. To take leg bail was old slang for "run away" (1774). Legs "ability to be an enduring success, staying power" is from 1970s show business slang.