Mama Luigi was an episode of the Super Luigi World T.V. show. This episode mainly stars Luigi, but Yoshi plays a very large role in it. In fact, the episode is about how Luigi found Yoshi.
Bowser found the key to Dinosaur World, and his Koopa Football Players grabbed the princess. Mario was chased away, by a Magnum Bill, and Luigi faced the Fire Sumo. The Fire Sumo cracked the earth open, sending Luigi falling. He found Yoshi's egg in a block, and the egg hatched. Luigi, while holding Yoshi, was chased away, by larger dinosaurs, and he went down a pipe. He ended up in the water, and the two of them were chased by a fish, as well as a Magnum Bill. They landed on dolphins, which took them to an island. Yoshi wandered away, but Luigi found him, just as caterpillars approached. Yoshi ate the caterpillars, just as Mario approached. (In fact, he even ate Mario, but he spit him out.) They found Bowser's palace, and Yoshi wandered away again, leading them to the throne room. Yoshi ate Bowser's robots, and took his key, just as the king of the Koopas made his escape. Yoshi spit the key into the lock, freeing the princess.
"Mama Luigi" was not left alone. Much of the episode's fame comes from Youtube poops, mashup videos that use dubbing. Luigi saying "Mama Luigi" has been constantly dubbed over episodes of T.V. shows, thus adding punchlines to the jokes. (It even sounds like "Mamma mia!")