I never used it before last year, but I never heard “Alt-right” before last year either.
The “alt-Left” is the mob that makes up their base (an alternative to the WASP norms - sexual deviants, Welfarians, unassimilated ethnics, etc.); they are the dupes of the Bolshevik “ctrl-Left”. This relationship was epitomized by that white lib b!tch asking for some “muscle” to deal with someone who disagreed with her (leading from behind, beckoning low-information slaves to do the dirty work)...