"This monk I speak of let old precepts slide" refers to the monk in the Prologue of The Canterbury Tales. Depending on the translation, the line occasionally appears as:
"This said monk let such old things slowly pace
And followed new-world manners in their place" (11-12).
The narrator's description of the monk seems full of admiration, yet satirically points out the corrupt practices of the Monk. For example, the bell on his horse's bridle jingles as "loud does the chapel bell," but ironically, the monk should be in the chapel helping people, not gallivanting around hunting on his horse. The narrator's praise of the monk's pleasant lifestyle and fine horse indicate that the monk has prioritized fine living over a humble life serving God.