Swoopfoomer is Doc's name for the type of party noisemaker that flares out and honks when blown into, as christened on Fraggle Rock.
In the first season episode "Don't Cry Over Spilt Milk," Doc realized that they had no specific name. Determined to give them one, he suggested "tromcordion," "foomaphone," and "pookarina." He also built a complex mechanized musical instrument out of several of them.
In the same episode, Red and Wembley gave one to Marjory the Trash Heap as an offering in exchange for her advice. She too remarked on their lack of a name.
The noisemakers did not appear again until the final season episode "The Honk of Honks," in which Sprocket got one in his cereal box. Doc referred to it as a "swoopfoomer," indicating that this was the name he had ultimately settled upon. The swoopfoomer turned out to be a key component in the Honk of Honks.
In Fraggle Rock: Back to the Rock, Boober's referee whistle has a swoopfoomer on the end.