The abbreviation KO (also K.O.) stands for knock out. It has its origin in the sport of boxing. Interestingly, in colloquial German, KO is often used in phrases such as
Ich bin ein wenig KO (= I am a bit exhausted)
Er ist ganz schön KO (= He is quite exhausted).
However, I don’t recall a similar use of KO in English, for instance in the form of
I feel really KO today
Instead, words such as tired, exhausted, or knackered are commonly used.
There are two interesting difference between KO in English and German. First, the English usage appears more consistent with the meaning of KO. Knocked out in boxing is an absolute state. (How does a bit knocked out feel compared to just knocked out?) The original meaning of KO may have been blurred when it became part of the German language, which does have degrees of being knocked out. Second, in English, the abbreviation KO is almost exclusively used in written English, and even then it is often spelled out (for example, knock-out price rather than KO price). In contrast, in German, KO is pronounced just like the letters K and O.