Sugar Free, Unsweetened
noun, の adjective
Something with not sugar is something that is sugar free or unsweetened.
むとう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
さっきも言ったけど、珈琲は無糖派なの。
As I told you earlier, I drink unsweetened coffee.