It’s obvious enough that a guiser is somebody who adopts a guise, thereby taking on a different form or appearance (disguise comes from the same root). It’s the usual name for the participants in local customs in Britain that involve dressing up and performing an entertainment, such as a mummers’ play.
One description of guisers appears in Yorkshire Folk-Talk, by the Rev M C F Morris in 1892. He said this about the mell supper (from a dialect word meaning “meal”), a traditional harvest-home supper in Yorkshire:
No mell supper can take place without dancing, and formerly the advent of “guisers” formed one of the great features of the entertainment. These “guisers” were men with masks or blackened faces, and they were decked out in all sorts of fantastic costumes. The starting of the dancing was not always an easy matter, but by degrees, as the dancers warmed to the work and as the ale horns came to be passed round, the excitement began to grow; this was increased by the arrival of the “guisers,” and then the clatter of the dancers’ boots doing double-shuffle and various comical figures, set the entertainment going at full swing.
It has survived most actively in Scotland. Reader Jane Brown wrote: “I lived in an Aberdeenshire village some 20 years ago and I remember the guisers who came round on Halloween night. They were the local youngsters, dressed up in ghoulish attire, who performed a song, dance or told a joke in exchange for sweets or a small amount of money. Before I left the area, the custom had begun to be taken over by the American Trick or Treat, which was not as popular with the residents”. And Chris Smith wrote: “In Shetland it’s always guizer. Children go guizing at Hallowe’en, but the most important guizing takes place at Up-Helly-A’ in late January. The torch-lit parade is led by the Guizer Jarl’s squad (who get to grow enormous beards and dress up as Vikings), followed by other squads of just plain guizers”.
The word has had other forms, such as guisard and another Scots form, gyser. A further dialect form, geezer, has become a common term for a man, as in diamond geezer, a London term of affection and admiration.