Buccus valdidus translates literally to "a strong goat". Bucco denotes a person who talks a lot and with not much sense; a chatterbox.
Judging by the place of this passus in Gregory's narrative, these events took place ca 588.
This abbot, most probably from Verdun as well, was probably a presbyter responsible for a basilica, rather than a monastic superior. More on presbyters described as abba, see Pietri 1983.