off- means away from, and road means the smooth made way that everybody normally uses. Put the two together and you get one picture: leaving that smooth surface behind and going onto whatever ground happens to be there — mud, sand, gravel, grass. Everything the word does comes from that step off the edge of the road.
off- 可以想成「離開」，road 是大家平常在走的那條鋪好的路。兩個字合起來，畫面就是：離開那條平整的路面，踩到旁邊原本的地上——泥巴、沙、碎石、草地。這個字後面的每一種用法，都像是從「跨出路面那一步」長出來的。
From the word’s history
Wiktionary gives the formation plainly: off- + road, with road going back through Middle English rode to Old English rād.
Wiktionary 直接寫明它的構成：off- 加 road。road 這個字則可以往回追到中古英文 rode、古英文 rād。
Wiktionary, entries “off-road” (adj and verb): “From off- + road.”
登入就可以看每一支怎麼連回核心畫面。