In the Diaspora the Shechitah knife is termed "Chalef".
What is the origin of this word? When was this word first used and where was this first first used?
A few possibilities.
Ezra 1:9 מחלפים. Rashi says that מחלפים is knives and it comes from בית החלפות where the knives were kept in the Bais Hamikdash.
Even Ezra and Metzudas David in Ezra 1:9 also translate מחלפים as knives. Metzudas David says the word is used for knives at you are changing the animal from life to death.
מחליפים את הבהמה מחיים למיתה
Bartenura Midos Chapter 4 Mishna 7 says the word comes from the Roman language.
שכן בלשון רומי קורין לסכינים הגדולים חלפים
Tiferes Yisrael Midos Chapter 4 Mishna 7 says the word comes from the Greek language.