Green’s Dictionary of Slang

combo n.2

[abbr. SE combination + -o sfx (6)]

1. [1920s+] a partnership, esp. a group of musicians.

2. [1920s+] (UK/US Und.) a combination lock (on a safe).

3. [1920s+] (US) any form of combination, whether of people, things, sandwich ingredients, wagers etc.

4. [1950s] a relationship.

5. [1980s+] (US campus) a bisexual person.