No, I had not heard it before either and I immediately thought of the word tryst. However, it turns out that there is no real connection and it in fact means melancholy or sad. As in,
“The tristful teenager fitfully tossed her dyed black locks over her shoulder and pouted as she waited in line to see the hot new vampire movie.”
“The events of the past year made Alice tristful indeed.”
This one could come in handy.
PS – Oh and in case you were wondering if I had written that fabulous title for this post I did not. Those are the words of one Ralph Waldo Emerson. Hence the quote marks.