Green’s Dictionary of Slang

rub(-out) n.

[rub out v. (1)]

(orig. US Und.) a murder, esp. an assassination.

[US]Hammett Glass Key 574: ‘Don’t be a sap, Neddy. This can’t get you anything but a rub-out. What good’s it going to do you to try to turn me up? None’.
[US]J. Archibald ‘No Place Like Homicide’ in Popular Detective Apr. 🌐 He [...] mentally crossed the holdup and rubout of a bank messenger off his memo pad.
[US]C.S. Montanye ‘Frozen Stiff’ in Popular Detective Mar. 🌐 You chalked it as a gang rub.
[US]H. McCoy Corruption City 77: An anonymous rub-out would most certainly have violent repercussions.
[US]C. Cooper Jr Syndicate (1998) 9: Those two-grand bumps where I had to give up maybe fifteen hundred [...] after the rub was over.
[US]D. Pendleton Boston Blitz (1974) 52: For Johnny and Val, it had to be a total rub-out.
[US]D. Jenkins Life Its Ownself (1985) 17: They could always hold gangland rub-outs there [i.e. a landfill].
[US]J. Stahl I, Fatty 238: William Desmond Taylor’s rubout and Mary Pickford’s divorce.
[US](con. 1973) C. Stella Johnny Porno 61: He heard the announcer say, ‘...a mob rubout in Queens’.

In compounds

rub-out job (n.)

(US und.) a murder, an assassination.

[US]B. Appel Tough Guy [ebook] Joey only used four or five for the rub-out jobs [...] guys with some brains.

In phrases

give somone the rub (v.)

(US) to murder, to assassinate.

[US]C. Cooper Jr Syndicate (1998) 23: My job [...] was to find the parties responsible for the job and give them the rub.