If something is "excruciating", it means that it's very, very painful. Use this word when you want to make something sound much more painful than just saying "very painful" or "extremely painful".
Because "very" is already included in the meaning of the word "excruciating", it sounds strange to say that something is "very excruciating". Adjectives like this are sometimes called "ungradable" or "extreme" adjectives. Here are some more examples:
The word "excruciating" can be used in these phrases:
an excruciating headache