mid-15c., a term in logic, "that which is said of a subject," from Old French predicat and directly from Medieval Latin predicatum, from Latin praedicatum "that which is said of the subject," noun use of neuter past participle of praedicare "assert, proclaim, declare publicly," from prae- "forth, before" (see pre-) + dicare "proclaim" (from PIE root *deik- "to show," also "pronounce solemnly," and see diction).
The grammatical sense of "the word or words which express what is said of a subject in a proposition" is from 1630s. Related: Predicative; predicator; predicatory.
word-forming element making adjectives from verbs, meaning "pertaining to, tending to; doing, serving to do," in some cases from Old French -if, but usually directly from Latin adjectival suffix -ivus (source also of Italian and Spanish -ivo). In some words borrowed from French at an early date it has been reduced to -y (as in hasty, tardy).
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Harper, D. (n.d.). Etymology of predicative. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/predicative