From advowry, variant of avowry, +‎ -er. Compare Middle English avouer (“patron, protector”), from avowen (“avow”), and avouwe, avowe (“patron, protector”), from Old French avoue.

advowrer (plural advowrers)

1. (obsolete, Early Modern) A protector, especially a patron saint.
   • 1507, John Estbury, “”, in John Footman, History of the Parish Church of Saint Michael and All Angels, Chipping Lambourn, London: Elliot Stock, published 1894, →OCLC, pages 73–74:

     and also I beseche our blissed lady seynt Mary w^t the speciall helpe of all the holy company of heven and of myn advowrers³ Petir and Pawle, seynt Frideswide, seynt barbara, seynt brigett, seynt Kateryne, & King Henry⁴ if he be soo at our lordes accepted, to be mediators for my soule, and all my frendes here in yerthe to pray for me.

   • 1527, Edward Echyngham, “The Will of Sir Edward Echingham”, in The Genealogist, volume 22 (N.S.), London: George Bell & Sons, published 1906, →OCLC, page 54:

     First I bequeth to almighty god and to the blissed Trinitie, to the blissed virgyn saint Mary our lady and to all saint and martirs, saint george saint Kateryn saint Antony myn advowerers and to all the blissed holy company in heven, most humbly requiryng the said Trinitie that they by the mediacion & intercession of myn advowrers that the company aforsaid will haue mercy and petie of my soule and conveye the same vnto the way of saluacion And my body to be buried before the Trinitie in Barsham chirch on the north side vnder the foundacion of the Chauncel wall.

   • 1532, John Fisher, Here after ensueth two fruytfull sermons ‎, fyrst Sermon, folio 8^v:

     To se there our speciall frendes and acquayntance which we had here before in this lyfe / to se the other sayntes whiche we dyd chuse for our aduowrers and patrons here in erthe, and whom specyally we dyd worshyp in this worlde.

• overdraw, wardrove