means A substance that speeds up activity in the body or nervous system, boosting alertness, energy, or wakefulness.

from From Latin 'stimulare,' to goad or prick, which itself comes from 'stimulus' — the actual word for a pointed stick used to drive cattle forward. The image is exact: a stimulant is a chemical jab in the ribs. The medical sense of a drug that 'pricks' the body into action arrived in the 18th century, but the goading goad had been poking livestock for two millennia first.