Green’s Dictionary of Slang

apple-knocker n.

[SAmeE apple-knocker, a fruit-picker; ult. origin may be in New York City, referring to the up-state, rural areas with their many apple orchards]

1. [20C+] (US) a rural, unsophisticated person.

2. attrib. use of sense 1.

3. [1920s+] (US tramp) an apple picker.

4. [1930s–40s] a fool.