I.H.S. Look up I.H.S. at Dictionary.com
Old English, from Medieval Latin, representing Greek abbreviation of IHSOUS "Jesus," in which -H- is the capital of the Greek vowel eta. The Roman form would be I.E.S. Mistaken for a Latin contraction in the Middle Ages, after its Greek origin was forgotten, and sometimes treated as short for Iesus Hominum Salvator "Jesus Savior of Men." Alternative version I.H.C. (terminal -s- often written in later Greek with a character resembling -c-) is found on vestments from 950 C.E., and may be the source of the H. in slang Jesus H. Christ.