documentary adj.

early 15c., "a doctrine;" late 15c., "teaching, instruction" (senses now obsolete), from Old French document (13c.) "lesson, written evidence" and directly from Latin documentum "example, proof, lesson," in Medieval Latin "official written instrument, authoritative paper," from docere "to show, teach, cause to know," originally "make to appear right," causative of decere "be seemly, fitting," from PIE root *dek- "to take, accept."

Meaning "written or printed paper that provides proof or evidence" is from early 18c., hence "anything bearing legible writing or inscription." Related: Documents.

adjective and noun word-forming element, in most cases from Latin -arius, -aria, -arium "connected with, pertaining to; the man engaged in," from PIE relational adjective suffix *-yo- "of or belonging to." The neuter of the adjectives in Latin also were often used as nouns (solarium "sundial," vivarium, honorarium, etc.). It appears in words borrowed from Latin in Middle English. In later borrowings from Latin to French, it became -aire and passed into Middle English as -arie, subsequently -ary.

"television drama based on real events," by 1957, American English, from documentary + drama. The first so-called appears to have been written as a stage play, "We Call to Mind," a "dramatic presentation of the development of education and its significance in American life," written by Philip C. Lewis and produced by the Tenafly, New Jersey, Citizens Education Council and the Tenafly Drama Workshop after the defeat of a school budget.