1620s, "bearing off or away," from French déférent (16c.), from Latin deferentem (nominative deferens), present participle of deferre "to carry down or away" (see defer (v.2)). Earlier in Middle English as a word in Ptolemaic astronomy (early 15c.) to explain the apparent motion of planets.
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.
<a href="https://www.etymonline.com/word/deferential">Etymology of deferential by etymonline</a>
Harper, D. (n.d.). Etymology of deferential. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/deferential