An arn was a measurement of time. ("I, E.T.) It was approximately equal to one standard hour on Earth.
When the crew was dealing with NamTar, John Crichton told Pilot that unless he volunteered one of his arms, Moya would be leaving in an hour. He then corrected himself, saying "one arn" ("DNA Mad Scientist").
Arns could be subidivided (e.g. a quarter-arn). ("That Old Voodoo")