late 14c., "unrestricted, free from limitation; complete, perfect, free from imperfection;" also "not relative to something else" (mid-15c.), from Latin absolutus, past participle of absolvere "to set free, acquit; complete, bring to an end; make separate," from ab "off, away from" (see ab-) + solvere "to loosen, untie, release, detach," from PIE *se-lu-, from reflexive pronoun *s(w)e- (see idiom) + root *leu- "to loosen, divide, cut apart."

Sense evolution probably was from "detached, disengaged" to "perfect, pure." Meaning "despotic" (1610s) is from notion of "absolute in position;" absolute monarchy is recorded from 1735 (absolute king is recorded from 1610s). Grammatical sense is from late 14c.

Absolute magnitude (1902) is the brightness a star would have at a distance of 10 parsecs (or 32.6 light years); scientific absolute value is from 1907. As a noun in metaphysics, the absolute "that which is unconditional or free from restriction; the non-relative" is from 1809.

1753 in theology, of God's actions; 1830 in political science, "system of government where the power of the sovereign is unrestricted," in which sense it seems to have been introduced by British reformer and parliamentarian Maj. Gen. Thomas Perronet Thompson. See absolute and -ism.

late 14c., "unconditionally, completely," from absolute (adj.) + -ly (2). From mid-15c. as "without reference to anything else, not relatively;" meaning "to the utmost degree" emerged by mid-16c. As a colloquial emphatic, by 1867, American English.

"Cannot something be done in the matter?" I inquired.

"Nothing sir! nothing, absolutely," he said (his family and personal pride evidently rising as he spoke); .... [D.E. Smith, "Leaves from a Physician's Journal," New York: 1867]

late 14c., dictatour, "Roman chief magistrate with absolute authority," from Old French dictator and directly from Latin dictator, agent noun from dictare "say often, prescribe," frequentative of dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly").

In Latin, a dictator was a judge in the Roman republic temporarily invested with absolute power; this historical sense was the original one in English. The transferred sense of "absolute ruler, person possessing unlimited powers of government" is from c. 1600; that of "one who has absolute power or authority" of any kind, in any sphere is from 1590s. 

1580s, "office or term of a (Roman) dictator," from dictator + -ship. The sense of "absolute authority" evolved by late 17c.