position

i don beliv it

Google is your friend

I guess it's because they generally occupy the center of the first-half of the pitch.

"The position is sometimes referred to as "centre-half". This originates the late part of the 19th century, when most teams employed the 2–3–5 formation, the row of three players were called half-backs. As formations evolved, the central player in this trio (the centre-half), moved into a more defensive position on the field, taking the name of the position with them."

Wikipedia