It probably comes from an Aramaic root meaning "to baptize," saba' (צבע).I tried the same trick on Masbotheans. There is an Aramaic name Masabala / Masbala / Masbalos. Couldn't find out what this means
Here is the passage from Hegesippus along with another one from the pseudo-Clementine literature:
Apostolic Constitutions 6.6: 6 For even the Jewish nation had wicked heresies, for of them were the Sadducees, who do not confess the resurrection of the dead, and the Pharisees, who ascribe the practice of sinners to fortune and fate, and the Basmotheans [Migne Βασμώθεοι, TLG Μασβωθαῖοι], who deny providence and say that the world is made by spontaneous motion, and take away the immortality of the soul, and the Hemerobaptists [Ἡμεροβαπτισταί], who every day, unless they wash, do not eat — nay, and unless they cleanse their beds and tables, or platters and cups and seats do not make use of any of them. And those who are newly risen among us, the Ebionites [Ἐβιωναῖοι], who will have the Son of God to be a mere man, begotten by human pleasure and the conjunction of Joseph and Mary. There are also those that separate themselves from all these, and observe the laws of their fathers, and these are the Essenes [Ἐσσαῖοι]. These, therefore, arose among the former people. And now the evil one, who is wise to do mischief, and as for goodness knows no such good thing, has cast out some from among us and has wrought by them heresies and schisms.
(Gah, rereading that just reminded me of an old pun from my evangelical youth to the effect that the Sadducees did not believe in a resurrection, and that is why they were so sad, you see?)
Anyway, back to the Masbotheans. Baptism in water seems to be a theme in a lot of these groups: the Essenes, Banus (reported by Josephus to have instructed in daily washings), the Sebuaeans (same Aramaic root as the Masbothaeans), the Hemerobaptists (whose very name suggests daily dippings), the Mandaeans (who cleanse themselves in water which they call a "Jordan"), and others.