Black Pepper

According to Wikipedia:

Black pepper (Piper nigrum) is a flowering vine in the family Piperaceae, cultivated for its fruit, which is usually dried and used as a spice and seasoning. The same fruit is also used to produce white pepper, red/pink pepper, and green pepper.[1] Black pepper is native to South India and is extensively cultivated there and elsewhere in tropical regions. The fruit, known as a peppercorn when dried, is a small drupe five millimetres in diameter, dark red when fully mature, containing a single seed.

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The spiciness of black pepper is due to the chemical piperine (an Alkaloid!).

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"Pepper" was used in a figurative sense to mean "spirit" or "energy" at least as far back as the 1840s; in the early 20th century, this was shortened to pep.