Green’s Dictionary of Slang

fiend v.

[fig. use of fiend n.]

1. [1980s] (US black) to steal, esp. in the street.

2. [1990s+] (US drugs) to suffering withdrawal from narcotics.

3. [1990s+] (US prison) to need intensely; to be addicted to.

4. [1990s+] (US black) to act aggressively.

In phrases

fiend (on) (v.) [1960s+] (US black)

1. to show off, to outdo a rival.

2. to covet, to lust after, to become obsessed with.