Green’s Dictionary of Slang

boss adj.

[boss n.2 (1)]
(orig. US)

1. [mid-19C+] superior, important, influential.

2. [mid-19C+] excellent, wonderful; also intensified as boss like hot sauce.

3. [mid-19C+] best; often as the boss, the best.

4. [late 19C] of criminals, very dangerous.

5. [late 19C] (US) arrogant, overbearing.

6. [1960s] dedicated, obsessed with.

7. [2000s] important, meaningful.

In compounds

boss player (n.) [player n. (3)]

[1960s] (US black) a thoroughly experienced, professional, worldly wise pimp who may even transcend pimping for superior occupations; the term can be applied to any admirable figure outside the pimp milieu.