Green’s Dictionary of Slang

(US) the point of eight in craps.

1898		St Paul Globe (Minn.) 28 Aug. 13/5: Each point on the dice has a certain name, which is known to every southern ‘nigger’ [...] Eight is [...] ‘eighty days’.
1908		Stell & Null Convict Verse 18: Eighty days an-ah great Big Dick!
1911		Howitzer 177: Big Dick from Boston—eighty miles from home—Phoebe—Little Joe—four’s my point...! [HDAS].
1919		Wilkins Co. Fund 45: Little Jo. Eighty days. Half of it. Shoes for the baby [HDAS].