Xmas is often thought of as a recent short form for people who are too lazy to write "Christmas" out in full. The real story is entirely different.
The form "Xmas" has been used since early mediaeval times. During the Middle Ages, scribes wrote largely on prepared animal skins, either vellum or parchment, in other words leather. Even today, leather is expensive and in those days books were made of sheets of leather. Therefore, it was important for abbreviations to be written in order to save space, and in fact the very calligraphy became compressed into the familiar "Gothic" or blackletter style we associate with "Ye Olde English".
X was a common abbreviation for "Christ", and is also found in, for example, Xtopher for "Christopher", and even survives today in circuit diagrams, where Xtal is written for "crystal", incorrectly as it happens. This is because the Greek for Christ is XPICTOC in Greek letters. The letter they write as "X" is chi, which is pronounced today as the ch in "loch".
"Jesus H Christ" reflects the Greek way of abbreviating "Jesus", which to us looks like JHC, so it's interpreted as his initials. The H in fact stands for the Greek long E, or eta. This abbreviation can be seen on tombstones and in churches.
Finally, i wanted to make this video on a train but i was too shy, and i walked backwards to avoid the wind ruining the sound, and because I'm walking backwards for Xmas.
I'm afraid that's it for today, i'm tired.