Bafflingly

As I tried to catch the words which the stoutly-fashioned floor so bafflingly intercepted, I was also conscious of a great deal of stirring and scratching and shuffling in the room below; so that I could not escape the impression that it was full of living beings—many more than the few whose speech I could single out.

To define is to specify, to trace limits; and whatever this 'something' is, it seems at once bafflingly specific and yet bafflingly without limits.