The Flyswatter is a nickname the Marshalls have given to the sharpened log they use on Grumpy during his unwelcome visits to High Bluff. It appears to be around ten feet long and eight inches in diameter (which would make it weigh in excess of 130 pounds, assuming that the wood used is similar to the conifers that existed even during earth's prehistoric past). The nickname is obviously intentional irony, as if a fully grown Tyrannosaurus rex were a mere nuisance, like a buzzing insect, rather than the deadly, territorial apex predator that it is.
Whenever Grumpy approaches the Marshalls' cave, his head right outside the entrance, the Marshalls pick up the flyswatter, wait for Grumpy to open his mouth (which he inevitably does to roar at the Marshalls) and then charge at Grumpy as if the flyswatter were a medieval battering ram, forcing the log into Grumpy's mouth and lodging it in his throat.
Grumpy, suddenly in danger of choking, invariably retreats as he struggles to dislodge the log. Returning to the jungle, he eventually manages to remove the flyswatter, then wanders off, apparently having lost interest in terrorizing the Marshalls. It is unknown if the Marshalls then retrieve the log or just keep replacing it. It also unknown if the recurring choking hazard has had any long-lasting effects on Grumpy's soft gullet.
Not being an intelligent animal, Grumpy never seems to learn from this abuse (at least not for long), and this scenario is played out repeatedly throughout the Marshalls' time in the Land of the Lost. However, Grumpy did once break off his pursuit of the Marshalls before they could administer it, which the Marshalls attributed to his finally remembering his previous experience with the flyswatter. If this were the case, however, Grumpy did not retain the lesson, as the Marshalls would be required to use it several times since then.
In every instance of the flyswatter's use, at least two of the Marshalls were employed to administer it. In its first recorded use, during the Marshalls first encounter with the Pakuni, it was used by Rick and Holly Marshall as Will was not present at the time. In later incidents, it was employed by all three Marshalls. It is known that Holly Marshall cannot manage it on her own. On one occasion, when Grumpy had Holly trapped in the cave alone, Will called to her from a safe vantage point outside the cave and suggested she use the flyswatter. Holly responded that she wasn't big enough to lift it by herself, and instead resorted to a red and yellow crystal combination, the blinding flash caused Grumpy to retreat.
There has been no instance of either Rick or Will attempting to use the flyswatter alone. It would likely be even more dangerous for a single person to attempt it, even if the person had sufficient strength to carry it while charging at Grumpy; a single person running with a ten foot log would also face the challenge of keeping it pointed directly at its intended target.
On one occasion, Rick Marshall suggested using "the ol' flyswatter trick on Emily," but this had nothing to do with the flyswatter the Marshalls use on Grumpy, nor was the intent to injure Emily or drive her away. Instead, it involved waving torches at Emily, causing her to instinctively back away from the flames in defense.
After the devastation to the Land of the Lost, the cave entrance to High Bluff permanently blocked and the Marshalls retreated to establish their home at The Lost Temple. Since it was located at ground level rather than at the height of Grumpy's head, the Marshalls were no longer under the threat of Grumpy's jaws outside the entrance to their home and retired the flyswatter altogether, leaving it at High Bluff.