Tendies means realised gains, as in securing the tendies. The word only ever refers to money taken out of the market, which is the one sensible idea buried in the joke.
It is worth keeping that distinction when the humour is stripped away. Open profit is a quote and nothing more; it becomes yours when the position is closed and the cash settles. See profit-giveback.
Related: profit-giveback, gain-porn, stonks, printing