early 13c. (as a surname), "a chopper, cutter," perhaps also "one who makes hacking tools," agent noun from hack (v.1).
Meaning "one who gains unauthorized access to computer records" is attested by 1975, and this sense seems to suggest hack (v.1), but the computer use is said to be from slightly earlier tech slang sense of "one who works like a hack at writing and experimenting with software, one who enjoys computer programming for its own sake," reputedly a usage that evolved at Massachusetts Institute of Technology (however an MIT student from the late 1960s recalls hack (n.) being used then and there in the general sense of "creative prank." This suggests rather a connection with hack (n.2) via the notion of "plodding, routine work." There may be a convergence of both words here.