The 60-year-old man told investigators he came across the words in a newspaper and wanted to look up their meaning, police spokeswoman Alice Casanova said.
"He teaches himself English by reading newspapers," she said. "It was all just a miscommunication."
The man was aboard United Airlines flight 1184 en route to Dayton, Ohio, on a business trip Sunday when a fellow passenger spotted the words and alerted an attendant, Casanova said.
The flight returned to O'Hare International Airport, where the man was taken into custody and all of the other 120 passengers were taken off the plane and rescreened.
"He was very calm when they walked him off the aircraft, very, very calm," a male passenger told CBS station WBBM-TV. "They handcuffed him and he just walked off the aircraft with the police."
The officers who arrested the Japanese passenger were not board the aircraft during the flight, a woman passenger told WBBM.
"About 4 it looked like U.S. marshals and local law enforcement here from Chicago-O'Hare P.D. came on board, and they went in the back, and he was sitting alone by this point," she said. "One gentleman leaned over and when they came back up the aisle, he was in handcuffs."
Investigators also searched the plane. "Nothing panned out and he was released," Casanova said.
Transportation Security Administration spokeswoman Andrea McCauley said travelers need to be mindful of how they behave on airplanes because potential security threats are treated very seriously.
"We caution people not to write about bombs because if they're going on vacation, their travel plans will be disrupted," she said.