I've said 'tisk' on occasions, in a kind of mock-fastidious way.

Some spellings acquire a life of their own. Unspellable noises get rendered by some hopelessly imperfect jumble of letters - tsk or tisk, miaow or meow, phew or whew, ugh, blech -; and later we're so used to the spelling convention that it becomes a word in its own right, and we pronounce it by its letters, not by the sound it originally represented.

Personally I've always thought ntk would be a better rendering.

In Zulu and Xhosa it's the sound of the letter c.