"Davka" is a word most Jews understand without necessarily being able to define it.
This is probably to do with the long, strange journey which the word has taken from its original roots.
As used today, "davka" means something like "just to annoy" (similar to "auftzuluchis"), or sometimes "in his own inimitable way." As in "She, ‘davka,' only wears green:" or "They drive on Shabbat, ‘davka:'" or sometimes even, "He'd be a nice guy if he wasn't so ‘davka.'"
It's fascinating to trace how "davka" comes to bear this variety of meanings. Its root is the Aramaic verb "duch," which means "to grind up fine," or "to turn into powder." From this it is used in the Talmud to mean "to examine something closely, carefully," or "punctiliously."
For example "lo duk" means "He did not express himself precisely." A "diyuk" is an inference derived from a close reading.
Based on this, the word "davka" is used in the Talmud to mean "precisely," "exclusively" or "only."
So, saying that matzah can be made "davka" from five types of grain means from those five and only those five.
Now we can see how a word meaning "precisely this way and no other way" comes to be used of behaviour which is inimitable or exactly calibrated to annoy.