Green’s Dictionary of Slang

(US) the Missouri River, Missouri.

1899		W.J. Kountz Billy Baxter’s Letters 70: He was from K.C., Mizzoo.
1905		Monroe City Democrat )MO) 5 Jan. 3/4: Oh Missouri, Mizzoo, I’m weeping for you.
1914		W.C. Handy Blues Treasury 88: I’ll...wash my hands in the Old Mizzou’ in K.C [HDAS].
1921		Columbia Eve. Missourian (MO) 28 May n.p.: California with all her enchanting scenery cannot overcome the lure of old Mizzoo.
1936		in E. Pyle Ernie’s America (1990) 141: He came [...] from Missouri. That’s why they call him Mizzoo .
1943		letter June 17 in R.R. Rea Wings of Gold (1987) 67: The river, ‘old Missou,’ is very high .
a.1963		in B. Ives Sing-along Song Book 154: The old Mizzoo, she’s a mighty river .