Originating from the webcomic "Darths & Droids", it describes an all-purpose spell in a role playing game, often used by a character in even the most ridiculous scenarios and situations.
Darths & Droids, Episode 33:
"I cast a spell to summon a bigger fish!" - "Like what?" - "Summon Bigger Fish!"

In a medieval RPG:
"You encounter a lonely deer, idly grazing away."
"I cast Fireball!"
"Oh no - why don't you just Summon Bigger Fish?!"
by ceejay33 January 10, 2012
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