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, meaning a “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.

Source: The Online Etymological Dictionary