"Stick a fork in it" (often followed by "it's done!") was an Earth idiom suggesting that a thing had reached completion or could no longer continue. It evoked the idea of testing a meat dish for thorough cooking by prodding it with a fork. If the meat had been rendered sufficiently tender, the fork would slide in easily.
When talking to Zotoh Zhaan about whether Maldis might return someday, John Crichton suggested that it would have been a good time for a reassuring lie. He rattled off a string of idioms for something (or someone) being completely finished, including: "Stick a fork in him! He's done! He's over! The fat lady is singing." ("Picture If You Will")