1725, "to breathe in," back-formation from inhalation or else from Latin inhalare "breathe upon," from in- "upon" (see in-(2)) + halare "breathe." Related: Inhaled; inhaling. Current sense is because the word was taken as the opposite of exhale. Slang sense of "eat rapidly" is recorded from 1924. As a noun, by 1934. Related: Inhaled; inhaling.