What is a slang word which originated during the and means nonsense?

You're likely thinking of the slang word "malarkey", which originated in the early 20th century and means nonsense, gibberish, or something insincere. It's often used humorously to express disapproval of something that seems ridiculous or untrue.

While the exact origin of the word is uncertain, it's believed to have emerged from Irish slang, possibly from "mallarkey" which referred to a type of playful or mischievous behaviour.