やだき(yadaki) is a more extreme form of きつい(kitsui). I've been warned that not everyone uses it, and even if people have heard it, this doesn't necessarily mean that they will remember the extact meaning or reference.
You might consider this like cult-slang. There are some expressions in British English that American people might not understand (there are plenty - let me assure you), but hearing the term, an American would react with "yeah that seems familiar but..."
That's kind of やだき for the masses. Use it when you are frustrated with some task, some person, a news item, or anything that generally causes tension. But it's a word that won't win you any friends in the upper-class. This would be like some really grumpy, crass old-man word that mostly younger boys use around high school and college.
Not a staple of the Japanese language - but that's the kind of thing I created this blog to document!