Sandwich slang

[normy]

why is a sandwich sometimes called a sarnie?

[shaneystar2]

Reg Varney ..sarnie ..Cockney rhyming slang.... could be one explanation.
Reg Varney was the actor who played Stan Butler in "On the Buses".

[Dollie]

Up North we sometimes call 'em Butties !!

[Blyss]

in scotland we call it a piece

[Quizmonster]

According to The Oxford English Dictionary, it is almost certainly just from the opening syllable of the word 'sandwich', given the long-'a' as in the northern dialect way of pronouncing it. The addition of an 'ee' sound is common in affectionate/diminutive word-forms...chappie, bookie, hankie etc. Put the two elements together and you get 'sarnie'.