Augustine (adj.)

"inspiring reverence and admiration, solemnly grand," 1660s, from Latin augustus "venerable, majestic, magnificent, noble," perhaps originally "consecrated by the augurs, with favorable auguries" (see augur (n.)); or else [de Vaan] "that which is increased" (see augment).

surname (also Austen) and masc. proper name, from Old French Aousten, an abbreviated form of Latin Augustine. St. Augustine of Canterbury is Austin in Layamon's "Brut" (c. 1200).

also -in, adjectival word-forming element, Middle English, from Old French -in/-ine, or directly from Latin suffix -inus/-ina/-inum "of, like," forming adjectives and derived nouns, as in divinus, feminus, caninus; from PIE adjectival suffix *-no- (see -en (2)).

The Latin suffix is cognate with Greek -inos/-ine/-inon, and in some modern scientific words the element is from Greek. Added to names, it meant "of or pertaining to, of the nature of" (Florentinus), and so it also was commonly used in forming Roman proper names, originally appellatives (Augustinus, Constantinus, Justinus, etc.) and its descendants in Romanic languages continued active in name-forming. The Latin fem. form, -ina, was used in forming abstracts (doctrina, medicina). Relics of the attempt to continue a distinction between Latin -ina and -inus account for the English hesitation in spelling between -in and -ine.