early 15c., "contiguous, bordering; close, nearby," from Latin adiacentem (nominative adiacens) "lying at," present participle of adiacere "lie at, border upon, lie near," from ad "to" (see ad-) + iacēre "to lie, rest," related to iacere "to throw; lay ('cast (oneself) down')," from PIE root *ye- "to throw, impel." Only of things, never of persons or animals. Adjacent, properly, is near but not necessarily in contact; adjoining is so as to touch. Latin adiacentia meant "the neighborhood."
