The story
From an Internet meme based on a screencap of Pikachu with a shocked expression from the 1997 episode "Bulbasaur and the Hidden Village" of the animated series Pokémon: Indigo League.
Etymology from Wiktionary, CC BY-SA 4.0.
shocked Pikachu
Today it means: A Pikachu with a specific open-mouthed, erect-ear expression signifying shock and astonishment, especially regarding something that one should have expected. Full definition →