early 15c., "slender, flexible rod for exploring the conditions of wounds or other cavities in the body," also "a medical 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, probably extended from the verb in this sense. Meaning "small, unmanned exploratory craft" is attested from 1953.
"Probe to the bottom," says President Roosevelt of the postal steals. Yes—"probe to the bottom," but don't overlook the top. What is needed quite as much as a probe—in fact, for the proper use of the probe—is a postmaster-general in the place of Payne, the mere partisan and convention fixer. [Chattanooga Daily Times, June 3, 1903]