The story
Blend of shemmy + cuck. Shemmy is a clipping of the imageboard's domain, soygem.party. "Sean" is the name of the board's mascot.
Etymology from Wiktionary, CC BY-SA 4.0.
shemmycuck
Today it means: A user of the imageboard soygem.party, nicknamed the Shemmy (a short-lived splinter of soyjak.party), especially one viewed as inferior, pathetic, or overly attached to the site by users of rival boards. Full definition →