The spelling of this word is one of those quirky differences between American and British English. In the US, the preferred spelling is "pajamas" while in the UK it's "pyjamas". Despite the fact that they're spelled differently, both versions refer to the same thing: a pair of loose fitting garments worn at night.
The word itself is actually derived from the Hindi term "paee jama", which translates to "leg clothing". The original pyjamas were made out of cotton and featured loose-fitting trousers and a long shirt. Today, they come in a variety of fabrics, styles, and colours. So whether you call them pajamas or pyjamas - you can be sure that they'll keep you snuggled up and cozy at night!
So there you have it: the history behind why we call them pyjamas. Now all that's left to do is pick some out and get ready for a good night's sleep! Enjoy!