The term ‘caribou’, when referring to the animal commonly known as a reindeer, was originally conceived as a bad pun. Technically, October 31st is the only day of the year when it’s proper to call these creatures by that name. The pun would be more obvious, but because it’s so groan-inducing and bad, the date restriction was lifted and spelling was changed in 1843 from ‘cari-boo’ to ‘caribou’ in an attempt to hide the shame of it all.
