"vigor, energy," 1912, shortened form of pepper (n.), which was used in the figurative sense of "spirit, energy" from at least 1847. Pep rally is attested from 1945; pep talk from 1926. To pep (something) up is from 1925.
例文
1. Powell spent the day giving pep talks to the troops.
パウエルは1日をかけて部隊のために動員演説を行った。
2.The Democrats held a pep rally on Capitol Hill yesterday.
民主党は昨日、国会の山で選挙決起大会を開いた。
3.I gave him an extra dose of glucose to pep him up.