Green’s Dictionary of Slang

kiddo n.

[kid n.1 (1) + -o sfx (1)]

1. [late 19C+] (also kiddoo) a general term of address to an adult, sometimes derog.

2. [20C+] a person.

3. [1910s] a girlfriend.

4. [1920s+] a young person or a child, esp. as a greeting, Hey, kiddo.