-tr-, -tero-, -ter, -dor
Relic of a comparative suffix.
A suffix in Proto-Indo-European and still used in Proto-Italic (cf. Spanish redor wre-tero-s "towards the back"). In Latin, the suffix was no longer productive but still identifiable, occuring in words like dexter, minister, and magister (whence derecho, maestro, and even madre).
From Proto-Italic *-tero- 'id.' From Proto-Indo-European *-tero- 'id.'
Italic: Oscan altrei "at another" (al-tero-), Umbrian putrespe "of both" (po-tero-kwid)
Indo-European: Celtic: Old Irish -thir, -ser, Old Welsh -ser, Middle Breton -zer, Old Cornish -ser; Germanic: Gothic -þar, Old Norse -tr, Old High German -der, Middle Dutch -ter, Old English -ðer (English -ther), Old Frisian -der; Balto-Slavic: Old Lithuanian -taras, Latvian -trs, Old Church Slavonic -terъ, Upper Sorbian -tery; Hellenic: Ancient Greek -τερός (terós); Indo-Iranian: Sanskrit -tará, Avestan -āra-