The story
Coined in 2012 on Tumblr. Clipping of the derogatory epithet “true transsexual scum”; by surface analysis, tru- (“true, genuine”) + scum. Some early instances also reference similar terms radscum and cis scum, which are possible influences. For additional context, see Citations:truscum.
Etymology from Wiktionary, CC BY-SA 4.0.