Pruning: Outlier Candidate Selection
I suspect this paper has typos (see e.g., line 2 of Algorithm 1).  If so, you're going to have to guess from context what they might have meant.
It's possible that they might have meant to write
$$fd_i = \frac{\sqrt{\frac{\sum_j (x_j - \bar{x})^2}{n}}}{\bar{x}}$$
In other words: the ratio between the standard deviation and the mean of that attribute.
Who knows; this is just a guess.  You might try contacting the authors.