A book containing either all or the principal words of a language, or words of one or more specified classes, arranged in a stated order, usually alphabetical, with definitions or explanations of their meanings and other information concerning them, expressed either in the same or in another language; a word-book; a lexicon; a vocabulary .... [Century Dictionary]
1520s, from Medieval Latin dictionarium "collection of words and phrases," probably a shortening of dictionarius (liber) "(book) of words," from Latin dictionarius "of words," from dictio "a saying, expression," in Late Latin "a word," noun of action from past-participle stem of dicere "speak, tell, say," from PIE root *deik- "to show," also "pronounce solemnly."
The Medieval Latin word is said to have been first used by Johannes de Garlandia (John of Garland) as the title of a Latin vocabulary published c. 1220. Probably first English use in title of a book was in Sir Thomas Elyot's "Latin Dictionary" (1538).
As an adjective, "of or pertaining to a dictionary," from 1630s. Dictionarist "compiler of a dictionary" (1610s) is older than dictionarian (1806 as a noun, 1785 as an adjective). Grose's 1788 "Dictionary of the Vulgar Tongue" has "RICHARD SNARY. A dictionary."
DICTIONARY, n. A malevolent literary device for cramping the growth of a language and making it hard and inelastic. This dictionary, however, is a most useful work. [Bierce]