The Italian verb "cacare" means "to shit". It's nowadays considered a vulgar word, but, judging from the examples of use from literary works of the Middle Ages quoted in the Grande dizionario della lingua italiana, it was not that way in Boccaccio's time. You can find it, for instance, in this medical book from 17th century. Undoubtedly Boccaccio uses it in this sentence to create a humorous effect.
In a note to his edition of the Decameron, Vittore Branca explains about the sentence said by the monk:
Parole senza senso, gettate là per confondere e stupire sempre piú Ferondo, come tante di quelle di Frate Cipolla: quasi a dire che le miglia sono tante da non poterle indicare con un numero.
Meaningless words, thrown there to confuse and amaze Ferondo more and more, like so many of the ones of Frate Cipolla: as if to say that there were so many miles that they couldn't be indicated with a number.
Curiously enough, the idea that these words are nonsense was already expressed in the first edition of the Vocabolario degli accademici della Crusca from 1612 (you can find here the contents of the entry "cacare" in this dictionary):
Qui è modo di favellare senza conclusione, e per parer di dire qualchè gran maraviglia, a chi non intende.
Here is a rambling way of speaking, to make the appearance of saying great things, to those who do not understand.
In the series of radio broadcasts produced by RAI to the integral reading of Boccaccio's Decameron, at the end of each episode there was a section called "Glossario", that is, "Glossary", in which the meaning of some words or expressions were explained. This is what it's said about the sentence said by the monk:
"sèvi di lungi delle miglia piú di be' la cacheremo": "sèvi di lungi" significa "vi sei lontano", ma il resto della frase non ha senso: è una sorta di provocazione per confondere Ferondo.
"sèvi di lungi delle miglia piú di be' la cacheremo": "sèvi di lungi" means "you are far away", but the rest of the sentence makes no sense: it is a kind of provocation to confuse Ferondo.
According to the Grande dizionario della lingua italiana this Boccaccio's expression, "essere lontano delle miglia più di ben la cacheremo", has been used as a set phrase with the scope of
indicare una distanza grandissima
that is, to indicate a very large distance.