Green’s Dictionary of Slang

1. (US, Western) ‘blackballed’, i.e. used of a ranch that was barred from sending cowboys to the general round-up or to work on other ranches [such exclusions were often made when ranchers were seen as being over-friendly to known cattle-rustlers].

1979

Maledicta III:2 162: siwashed adj Cowboy’s term for blackballed; siwash is a nickname for any Indian, from the French sauvage meaning ‘savage, wild, uncivilized’.

2. (US) to ban someone from buying alcohol; thus siwashing n.; siwashed adj.

1910		Bulletin (Sydney) 13 Oct. 18/4: They call this scheme ‘siwashing’ over there [i.e. the US], and its advantages are that it does away with the old, ponderous formality of trial or judicial investigation, and allows the brisk and fair-minded Rechabite or holy-Willie to be informer, prosecutor and practically judge. [...] Incidentally, it destroys a lot of liberty; but when the wowser comes in at the door liberty is thrown out the window.
1910		Vancouver Dly World 18 May 6/2: In a recent prosecution for supplying liquor to a ‘siwashed’ man [...] the accused saloonkeeper was fined $25.
1929		Willoughby Trail Eater 183: I’m going to have you siwashed. [...] I’m going to the judge tonight and fix it so you’ll never get another drink in this town [DARE].
1930		Star Trib. (Minneapolis MN) 3 May 16/4: The logger who cannot handle his likker [...] may be siwashed.
1954		Baytown Sun (TX) 17 June 6/1: Old Dan is 24, and that’s pretty old for a horse. His favorite hang-out is a bar [...] A railroad man [...] decided it was time the sway-backed nag was siwashed — an indian word that means ‘that’s all, brother, you’ve had enough to drink’.
1958		J.F. Adams ‘Ranching Terms’ AS XXXIII:4 272: siwashed, to be; on the indian list, to be. To be blackballed. Liquor stores keep (or did keep) lists of Indians who attempted to buy liquor, as well as the names of liquor offenders. This was known as the ‘Indian list’ and seems to be the origin of the expression.
1967		in DARE.
1970		(ref. to late 19C) Fairbanks Dly News-Miner (AK) 26 Jan. 2/1: To be ‘siwashed’: At he time of the gold rush it was unlawful to see hard liquor.