"the black liquor with which men write" [Johnson], mid-13c., inke, from Old French enche, encre "dark writing fluid" (12c.), earlier enque (11c.), originally enca, from a shortening of Late Latin encaustum, from Late Greek enkauston. This is the neuter of the past-participle adjective enkaustos "burned in," from the stem of enkaiein "to burn in," from en- "in" (see en- (1)) + kaiein "to burn" (see caustic).
In Pliny the word is the name of a kind of painting method involving fire or heat.
It denoted a kind of painting practised by the ancients, in which the crayon was dipped in wax of various colours.
Encausto pingere is to practise this art, paint in encaustic or enamel. [footnote in "The Life, Letters, and Sermons of Bishop Herbert de Losinga," Oxford, 1878]
Later it was the special name of the purple-red ink, the sacrum encaustum, used by the Roman emperors to sign their documents; this was said to have been obtained from the ground remains of certain shellfish, formed into writing fluid by the application of fire or heat, which explained the name. In the Code of Justinian, the making of it for common uses, or by common persons, was prohibited under penalty of death and confiscation of goods.
The usual word for "ink" in Latin was atramentum (source of Old French arrement), literally "anything that serves to dye black," from ater "black;" the Greek word was melan, neuter of melas "black." The Old English word for it was blæc, literally "black," and compare Swedish bläck, Danish blæk "ink." Spanish and Portuguese (tinta) and German (tinte) get their "ink" words from Latin tinctus "a dyeing."
Donkin credits a Greek pronunciation, with the accent at the front of the word, for the French evolution; the same Latin word, behaving regularly, became inchiostro (with unetymological -r-) in Italian, encausto in Spanish.
As an adjective, inken (c. 1600) occasionally has been used. Ink-slinger, contemptuous for "journalist," is from 1870. The psychologist's ink-blot test attested from 1915.