cuz:
- short for "because"
- short for "cousin"
- something else, see Wikipedia:Cuz
Most of the following data is from https://googlebooks.byu.edu/#.
As in "because" 
Edit
From later to earlier:
- 2016: http://archived.moe/sci/thread/7816285
- 1972: https://books.google.com/books?id=lE0gAQAAIAAJ&q=%22cuz%22
- 1891 (written as 'cuz): https://books.google.com/books?id=OW0CAAAAIAAJ&q=%22cuz%22
- 1885 (written as 'cuz): https://books.google.com/books?id=fbsuDwAAQBAJ&pg=PA84#v=onepage&q=%22cuz%22&f=false
- 1865: https://books.google.com/books?id=WWMVAAAAIAAJ&q=%22cuz%22
- 1854: https://books.google.com/books?id=mkDhr45qVqIC&q=%22cuz%22
As in "cousin" Edit
From later to earlier:
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