Hannah
As the most popular motivational speaker, all-time best selling author, and best-ever-birthday-party-thrower, Jesus Christ is undoubtedly the most famous person to ever exist. Many people use his name as an expletive but sometimes a simple “Jesus Christ” isn’t enough. Occasionally you need one more letter to make your point and you yell “Jesus H. Christ” like an angry parent using their child’s full name. But how did the “H” enter Jesus’ OG Chrisitan name? Is it Henry, Holy, Hard-ass?
Well, there are two monograms used to represent Jesus Christ’s name. The first is the Chi Rho monogram, which consists of the first two letters of the Greek word Χριστό, chi ⟨Χ⟩ and rho ⟨Ρ⟩, meaning Christ.
The second monogram is the IHC monogram. It consists of the first three letters of Ἰησοῦς (Iēsoũs), which is the Greek spelling of the name “Jesus.” 
It will come as no surprise Americans looked at this glyph in the early 19th century and misread Greek letters for their own Latin letters. They read the letters “IHC” as “JHC.” These very sharp Americans took this to mean the “J” stood for Jesus, “C” to stand for Christ, and the H to stand for Jesus’ newly discovered middle name. Luckily for you, Latin is basically a dead language so you can pronounce these however you’d like now.
From there, the phrase “Jesus H. Christ” became a humorous expletive even used by Mark Twain in his autobiography in the 19th century and by me, when I’ve definitely checked to make sure my mom isn’t within earshot.
So that’s it! Jesus H. Christ all started as one of the many American mistakes. Luckily, we now have a great expletive and you are free to say the H stands for whatever you’d like.