Green’s Dictionary of Slang

plug n.3

[Du. plug, a worn-out horse]

1. [mid-19C] (Aus./N.Z.) a sturdy horse, standing about 15 hands high, that is sufficient for the work required.

2. [mid-19C–1940s] (US) an incompetent or undistinguished person; also attrib.

3. [mid-19C+] (US) a worn-out old horse.

4. [mid-19C+] (US) a fellow, a person, a chap.

5. [1900s] (US campus) a hard-working student.

6. [1900s] (Can.) an unpleasant person.

7. [1910s] (US) a hard-working but materially unsuccessful person.

8. [1930s] a worn-out racing greyhound.

9. [1940s+] (US) a damaged or malfunctioning object, e.g. an old car.