this is one of the many tone indicators used predominantly on twitter (though some people also use them in regular online messaging). it means “teasing,” and you put it at the end of a sentence to indicate your words were meant in a teasing way, and to not take them too seriously. it’s similar to /pos (positive) and /gen (genuine), which are also common indicators on twitter.

the whale tail is the shape formed when a g-string rides up high over a womans trousers