ow
Synonym discussion
Sense: exclamation/1
For a quick, sharp pain, 'ow' is one of the shortest and most direct reactions in English. 'Ouch' is very close, but it sounds a little fuller and is often used when the speaker wants to show the pain more clearly. 'Oof' often follows a hit, fall, or heavy landing, so it mixes pain with shock. 'Ugh' is different because it usually shows annoyance, disgust, or tired frustration, not a sudden sting in the body. 'Yikes' mainly shows alarm or nervous surprise. If you hit your knee on a chair, 'Ow!' fits naturally. If a friend falls hard off a bike, 'Oof!' may sound more natural.
Etymology
'Ow' is an imitative English pain cry, written to match the sound people make when something hurts suddenly. English has used spellings of this natural reaction for a long time, and the word did not grow from an ordinary noun or verb root in the way many other words did. The abbreviation sense is much later and simply shortens 'one-way' in travel and booking contexts.