It is common in online conversation among some Internet users to use a fictitious closing tag patterned after HTML: . Over time, it has evolved to lose the angle brackets (/sarcasm) and has subsequently been shortened to /sarc or /s (not to be confused with the valid HTML end tag used to end a struck-through passage).[20] Users of the website Reddit frequently denote sarcasm through the use of /s, as shorthand.[21] This usage later evolved into tone indicators.

Wow, I didn't know the /s had an origin story. Pretty cool.