1550s, "to point out, indicate, exhibit," a sense now obsolete, 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 or establish the truth of by argument or deduction" is from 1570s. Sense of "describe and explain scientifically by specimens or experiment" is from 1680s. Meaning "take part in a public demonstration in the name of some political or social cause" is by 1888. Related: Demonstrated; demonstrating.
Latin also had commonstrare "point out, reveal," praemonstrare "show beforehand, foretell."