“Challenge Accepted” is an expression used to indicate that the speaker is willing to attempt completing a difficult task. The phrase is often associated with a rage comic character illustrated as a stick figure with crossed arms and a smug facial expression.
“Challenge accepted” began as a catchphrase for the character Barney Stinson in the television sitcom How I Met Your Mother. The earliest instance of Stinson uttering the phrase can be heard in Season 2, Episode 6 “Aldrin Justice” while announcing his intention to seduce an older woman. The episode originally aired on October 23rd, 2006
Barney Stinson: Marshall, I’ve thought it over and I accept your challenge.
Marshall Eriksen: I didn’t challenge you to have sex with…
Barney Stinson: Tomorrow, the cougar hunt begins.
The earliest known instances of the the rage comic character were posted in a thread by Body Building Forums member MK on November 12th, 2010, which included two challenge accepted examples (shown below).