We've found this word Abetrours here and here, but cannot seem to find a definition of it. Can anyone refer us to one, and ideally its etymology as well? Feel free to suggest another StackExchange site (Linguistics?) if this isn't the appropriate one. Thank you kindly!
The 1971 OED does not include an entry for abetrour; but matches for it, in sigular or plural form, appear six times in Google Books search results. In each case, however, the match turns out to be a false positive, and the actual word on the page is either abettour or abettours. Here, in chronological order, are those six instances, with links to an image of the original text page where each appears.
From Richard Bernard, The Isle of Man: Or, the Legall Proceeding in Man-shire Against Sinne (1627):
Thou hast heere towards the end of this discourse, the tryall and iudgement upon foure notorious Malefactors. Two of them the very prime Authors of all the open rebellion, or secret Conpiracies, which at any time euher were in that Iland: The other two were the principall Abettours and the chiefest Supporters of them.
From John Lesly, An Epithrene: or Voice of Weeping: Bewailing the Want of Weeping (1631):
It is in a manner all one, to commit and to commend a Villany. But Divines say more, Non caret scr[?]upulo occultæ Societatis, qui manifesto discrimini non occurrit ; Hee is suspected to be an Abettour of evill , who endevoureth not to abandon evill.
From a translation of John Prideaux, The Doctrine of the Sabbath: Delivered in the Act at Oxon, Anno, 1622, third edition (1635):
Yet notwithstanding all this care, both generally of the Apostles, and more especially of S. Paul, to suppresse this errour, it grew up still, and had its patrons and abettours. Ebion and Cerinthus, two of the wretchedst hereticks of the primitive times, and after them Apollinaris, are said to countenance and defend it ; which doubtlesse made the ancient fathers declare themselves more fully in it, as a dangerous point; which seemed to confirme the Iewes in their incredulity and might occasion others to make quest of our Saviours comming in the flesh.
From Gerard Langraine's 1638 translation of Guillaume Ranchin, A Review of the Councill of Trent (tr. 1638):
'With the approbation of the Councell we condemne, reject, detest, and declare to be void, invalid, and of no effect, all the acts, facts, gests and writings published and ordained by the children of damnation, Bernardin Caravaial, William Brizonnet, Renald de Pria, and Frederic de Saint Severin, heretofore Cardinals, together with their favourers , abettours, and complices, Schifmatiques and Heretiques : who endeavour to breake the union of the holy mother Church by the Conventicles of Pisa, Milan, and Lyons. ...'
From John Canes, Fiat Lux: Or, A General Conduct to a Right Understanding in the Great Combustions and Broils about Religion Here in England Betwixt Papist and Protestant, Presbyterian and Independent (1665):
The Works, whose merit S. Paul disables there, were apparently such as were done before conversion, of which the abettours would have by those Works to be the cause, Works acted in Judaisme and and Paganisme, without Christ who reconciled the world to God, without the assistance of his Grace, without his Command, without his promise of reward for them, and consequently without any value or acceptablenesse they might have upon those grounds.
And from John Brand, A New Description of Orkney, Zetland, Pightland-Firth, and Caithness (1703):
The Countrey People here did and do think that the Captain of the Ship willingly suffered her to drive upon this point, and the Men there to Perish, and if so, it is probable that others tho not Aboard, have been concerned in this mischievous desing [that is, design], as the Author and Abettours thereof.
All six matches for abetrours in the Google Books search results are thus misreadings by Google Books' OCR software of the word abettour[s] as abetrour[s].
Early English Books Online does not provide photo images of the pages of the books it provides transcriptions of, so I can't offer visual evidence that the instance cited in the posted question is likewise an OCR error. However, a couple of points provide further circumstantial evidence that this is the case.
First EEBO reports only two instances of abetrour[s] in its database of texts from 1475 to 1700. They are as follows.
From Serenus Cressy, Exomologesis, or, A Faithfull Narration of the Occaision and Motives of the Conversion unto Catholick Unity of Hugh-Paulin de Cressy, Lately Deane of Laghlin &c. in Ireland and Prebend of Windsore in England (1653):
But as for the Opinion of Waldensis, it ha's found many abetrours in these latter ages, for Fr. Pious Mirand•la in his eighth Theoreme de Fid. & Ord. •red: saith, Those Decrees may justly be ••lled tho the Decrees of the universal Church, which are either made by the Pope the Head thereof, or by a Councell, in which the Church is represented in matters necessary to Faith, and which are approved by the Church her self.
This is the instance cited in the posted question the instances of "•" and "••" in the quoted passage represent places where the OCR software found the lettering unintelligible, meaning that the original page here may have been somewhat difficult to read clearly—although the fact that they occur in italicized text, while the reading of abetrours appears in roman text, is noteworthy.
And from Samuel Fisher, Rusticus ad Academicos in Exercitationibus Expostulatoriis, Apologeticis Quatuor The Rustick's Alarm to the Rabbies, or, The Country Correcting the University and Clergy, ... (1660):
The more shame for thee I.O. if the Quakers be all so unlearned, and utterly unintelligent in the Latine Tongue (as thou sayest) that thou talkest therein against them (as thou dost) and chargest them with much more error in Doctrine and evil in life, then will ever be made good against them by thy self or any of thine Abetrours or stand approved for Truth, while the world stands among spiritually understanding and honest minded men, when they come to be divested (as hereby they are to be) our of that disguise thou dressest them our in to thy Iunior Ieerers at Christs own Image, which is seen upon them.
In contrast, an EEBO search yields 213 matches for abettour[s], ranging in date from circa 1502 to 1699, including all five books from that time period that the Google Books searches found, with abettor[s] rendered correctly in each case. This suggests that (1) the EEBO OCR software is better than the Google Books OCR software; (2) it nevertheless makes mistakes; and (3) abettour[s] was a not uncommon spelling of abetter[s]/abettor[s] in the sixteenth and seventeenth centuries.
Since the sense of "abetrour[s]" in every instance quoted above makes sense when read as abettours, I think it is fairly safe to conclude that the instance of abetrours cited in the posted question is either an OCR error (albeit one picked up by two different website renditions of the text) or a typographical error in the original text. In either case, the word intended was evidently abettours.