demonstrate (v.) Look up demonstrate at Dictionary.com
1550s, "to point out," from Latin demonstratus, past participle of demonstrare "to point out, indicate, demonstrate," figuratively, "to prove, establish," from de- "entirely" (see de-) + monstrare "to point out, show," from monstrum "divine omen, wonder" (see monster).

Meaning "to point out by argument or deduction" is from 1570s. Related: Demonstrated; demonstrating. Latin also had commonstrare "point out, reveal," praemonstrare "show beforehand, foretell."