Well, the spelling depends on whether Guest is in the UK of the US.

Sam Johnson was given a double dose of sorts for the letters S and U, but the molds for the Cs and Zeds broke, so Johnson gave it up and used the S for most of whatever he might have otherwise spelled with a C or Zed, and tossed an extra U in, hither and yon, for the fun of it.

Meanwhile, the fellow making sorts for Webster ran amuck with Cs and Zs, causing an overflow in the US. Webster had his own kind of fun with that.

Whaddya mean, theory? I'm so old, I WAS THERE!

Give each other a break...

Sarah