early 15c., "instrument for exploring wounds, etc.," also "an examination," from Medieval Latin proba "examination," in Late Latin "a test, proof," from Latin probare "show, demonstrate; test, inspect; judge by trial" (see prove). Meaning "act of probing" is 1890, from the verb; figurative sense of "penetrating investigation" is from 1903. Meaning "small, unmanned exploratory craft" is attested from 1953.