In The Apple Tree

pep (n.)

“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.

From The Online Etymological Dictionary