Just wondered if anyone could add to this.
It first appears in the Savitars in the one published in 1907. It was used in the Missouri Alumni Quarterly as early as December 1905.
I found the following timeline:
The first football game, with MU students seated on both sides of the stadium, was played on Thanksgiving Day. The Savitar reports “3,000 fans cheered "M-I-S-S-O-U-R-I-Ah!”
A Savitar article about the football team ends with a cheer, “Missou-rah-rah.” This is the earliest MU-sanctioned document to use the word “Missou.”
The Missouri Alumni Quarterly and the Savitar print the word “Mizzou,” the earliest recorded appearance of MU’s nickname.
The MU-KU first-ever Homecoming game. Mizzou’s rivalry with Kansas dates back to the Civil War, but this game was the first time Kansas played in Columbia. This rivalry begat the popular parody of our current chant: “M-I-Z beat K-U” or for some, “M-I-Z F K-U.”
“Old Missouri” and “Old Missou” are adopted as unofficial nicknames for the University in the Savitar archives. The poem “Spirit of Old Missou” is printed in the 1917 Savitar.
The Mizzou Razzers are founded. This is the first spirit group that claims “loyalty to Missouri” and that “Missouri is first in all things,” including “victory with honor.”
The Savitar lists the cheerleaders’ cheers, including “The Varsity,” a chant that is strikingly similar to today’s “Every True Son” in its borrowing of the phrase “M-I-Z-Z-O-U! Rah!-Rah!-Rah!-Rah!”
Seth Rosner, one of the co-founders of Tiger’s Lair, believes the cheer was adapted from Ohio State University’s “O-H-I-O” chant just after MU played the team in the late ’70s. Missouri starts an echoing chant of its own: “M-I-Z,” “Z-O-U.” Ohio might have been first, but Mizzou’s cheer has a better ring to it.
Missouri Alumni Magazine changes its name to Mizzou in the fall of 1995 and claims the new name is “warm, affectionate and unique.”
Just Mizzou It! Quin Snyder becomes the men’s basketball coach and lands a contract with Nike. This is the start of “Mizzou-ification,” which refers to the commercialization of the Mizzou brand.
The football team jumps on the Mizzou bandwagon, and football jerseys switch from a small “Missouri” above the numbers to “Mizzou.”
"In 1911, MU held the first Homecoming at the football game between MU and the University of Kansas. After World War II universities around the country grew at an extraordinary pace and MU was no exception. This was due, in part, to the G.I. Bill which allowed veterans to attend college with the assistance of the federal government. It was also during this time that the nickname "Mizzou" was first used. The usage of the term "Mizzou" was first recorded in a campus yell that used the phrase "Mizzou, Rah, Rah." The commonly accepted origin is that the word is a shortened version of the University's (then-known as the Missouri State University) initials, MSU. When said quickly the initials can be morphed into the affectionate nickname: Mizzou. The name stuck and now is commonly used interchangeably among students, alumni and the residents of Columbia with the newer initials, MU."