Other forms: serges
Serge is a strong fabric that's usually made from wool and is often used in military uniforms. The way serge is woven creates diagonal ribbing on the front and back.
Serge sounds just like surge, but the two words have entirely different meanings and derivations. The material serge is from the Medieval Latin sargium, "wool cloth mixed with silk," and a root meaning "silken." You probably wouldn't describe modern serge as "silken," though — the sturdy fabric is actually famously rough and itchy, especially when it's worn next to the skin.