According to one source, the expression originally comes from racing, used in its most literal sense. Another source claims that it has its roots in 明代, in a family of expressions including "火上加油". Both agree, though, that "加油" became a general expression of encouragement though its metaphorical usage becoming more and more closely tied to the word by itself. There is nothing in the meaning of the constituent characters themselves, however, that conveys the meaning of encouragement. Not all Chinese words can have their current meaning explained in terms of their parts (like 肥皂).

Source alleging origins in racing: http://zhidao.baidu.com/link?url=yiP6iAc0S-3VMztyCR8dID7OYfAskDbfmR8_jBZ0Zs9RXweUeYBLpzRYnNBNU5PyNcOLgyrxMKTaWGb49G3Xpa Source alleging origins in 明代: http://www.xiexingcun.com/yuwenjianshe/ywjs2009/ywjs20090125.html

According to one source, the expression originally comes from racing, used in its most literal sense. Another source claims that it has its roots in 明代, in a family of expressions including "火上加油". Both agree, though, that "加油" became a general expression of encouragement through its metaphorical usage becoming more and more closely tied to the word by itself. There is nothing in the meaning of the constituent characters themselves, however, that conveys the meaning of encouragement. Not all Chinese words can have their current meaning explained in terms of their parts (like 肥皂).

Source alleging origins in racing: http://zhidao.baidu.com/link?url=yiP6iAc0S-3VMztyCR8dID7OYfAskDbfmR8_jBZ0Zs9RXweUeYBLpzRYnNBNU5PyNcOLgyrxMKTaWGb49G3Xpa Source alleging origins in 明代: http://www.xiexingcun.com/yuwenjianshe/ywjs2009/ywjs20090125.html

From what I can gather, the expression originally comes from racing, used in its most literal sense. The expression "加油" then became a general expression of encouragement. There is nothing in the meaning of the constituent characters themselves, however, that conveys the meaning of encouragement. Not all Chinese words can have their current meaning explained in terms of their parts (like 肥皂). I found a simple summary of the history of the word here: http://zhidao.baidu.com/link?url=yiP6iAc0S-3VMztyCR8dID7OYfAskDbfmR8_jBZ0Zs9RXweUeYBLpzRYnNBNU5PyNcOLgyrxMKTaWGb49G3Xpa

According to one source, the expression originally comes from racing, used in its most literal sense. Another source claims that it has its roots in 明代, in a family of expressions including "火上加油". Both agree, though, that "加油" became a general expression of encouragement though its metaphorical usage becoming more and more closely tied to the word by itself. There is nothing in the meaning of the constituent characters themselves, however, that conveys the meaning of encouragement. Not all Chinese words can have their current meaning explained in terms of their parts (like 肥皂).

Source alleging origins in racing: http://zhidao.baidu.com/link?url=yiP6iAc0S-3VMztyCR8dID7OYfAskDbfmR8_jBZ0Zs9RXweUeYBLpzRYnNBNU5PyNcOLgyrxMKTaWGb49G3Xpa Source alleging origins in 明代: http://www.xiexingcun.com/yuwenjianshe/ywjs2009/ywjs20090125.html