https://en.wikipedia.org/wiki/Christogram
(eastern church)
Richard,
I think Denise is right. It means Jesus Christ (iesus christos) - old manucripts used the first and last letter of a name of god and a dash over it (such as ts for theos). Here you have iota and closing sigma (s) for iesus=Jesus and Chi - the Ch of christ - and again closing sigma (s) for christos=Christ with a tilde symbol standing over both to tell it's an abbreviation.
The technical name for those is "nomina sacra". It's a fascinating apologetic point for which books are canonical.
