Translation for this site. The original is "Secundum miraculum; ostium Trans Hannoni fluminis, quia in una unda instar montis ad sissam tegit litora, et recedit, ut cetera maria."
The original's ad sissam is obscure. Morris* translated this as "in the bore", and this fits with its use elsewhere in the text (Wonder 11), but sadly he left no indication of where this translation came from. Rhys* translates the same word in Wonder 11 as "banks", but this may be hard to reconcile with the use of litora ('banks', 'shoreline') here, unless it is "to the banks it covers the shores". Conversely Koch and Carey* for the same text seem to assume it is the name of a river ("to the Sissa"..."floods the estuary of the above-said river").While doubt is thrown on Morris' translation by the fact that it seems the Irish Nennius don't mention the bore at all, the translator may have simply ditched a word they didn't recognised. Färber & Irwin* have the translation "The second wonder is the mouth of the stream Tranon, which is filled from the bottom with one wave, and ebbs like every other sea." Later transcribers of the non-Irish Historia appear to have dropped the term in Wonder 6: see Note on Alternatives.
It is not clear whether there are other Latin texts noting a bore explicitly, which might give a usage. Chanson* suggests that the Seine Mascaret was noted in the 7th C but his reference is hard to obtain. It is possibly the mention of the tides in Dudo of St. Quentin's Gesta Normannorum (Chapter 10) which relates events from 876 CE, but doesn't give the feature a name. Note, also, the writer uses ostium (river mouth) rather than aestuarium (tidal estuary).
The word doesn't seem an adaptation of Welsh. The Welsh for bore is now eger, however, in Culhwch ac Olwen (c.1325 CE*) we find pan delhei Arthur a'e luoed y uron llifdwr[*line 279] ("when Arthur and his troops came to a cresting flood" [...a giant knife would be used as a bridge]*p.145), so it is possible there was no Welsh word for the bore in the Middle Ages, if not before. It seems likely that eger has been picked up from the English "Aegir" (after the Norse god), which is now used only in English for the Trent bore.
However, for the sake of this translation, sissam has been assumed to be scissam "teared"; "splited"; "divided". The word also appears in Wonder 6, and in Wonder 11 in sissa Sabrinae, which is translated assuming scissa (again "teared": "the teared Severn"). This assumption has a precedent closer to the Historia's period, as it used in a summary of the wonders by Lambert of St Omer (See Other Lists). It is to be assumed that "teared" is a reference to the bore cutting the river into two sections along its rough-cut line, or similar.
NB: With this assumption of scissa in mind, it is perhaps worth noting that in the Hunt for the Trwch Trwyth from the medieval tale of Culhwch and Olwen (Details), the "scissors" of most translations (modern Welsh siswrn) are removed from the boar at the Severn, however, the term used consistently is actually gw(/u)elleu(/iu) ("shears": modern Welsh gwellau). Note also that the Latin for scissors used in the Historia (Section 39) is forcipe (cf. more standard forfex). It is therefore unlikely the removal of the scissors is related, at least by name, to the bore – though there may be a less explicit link.