Green’s Dictionary of Slang

one-eight-seven v.

also one eighty seven, 187
[one-eight-seven n.]

(US black gang) to murder; thus gang slogan used by the Crips of Los Angeles B187, ‘we kill Bloods’.

[US]L. Bing Do or Die (1992) 198: B-one-eight-seven means we kill Bloods.