"vigor, energy," 1912, shortened form of pepper (n.), which was used in the figurative sense of "spirit, energy" from at least 1847. Pep rally "meeting to inspire enthusiasm" is attested from 1915; pep talk is from 1926. To pep (something) up "fill or inspire with vigor or energy" is from 1925.
-y 2
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.