late 13c., peine, "the agony suffered by Christ;" c. 1300, "punishment," especially for a crime, "legal punishment of any sort" (including fines and monetary penalties); also "condition one feels when hurt, opposite of pleasure," including mental or emotional suffering, grief, distress; from Old French peine "difficulty, woe, suffering, punishment, Hell's torments" (11c.), from Latin poena "punishment, penalty, retribution, indemnification" (in Late Latin also "torment, hardship, suffering"), from Greek poinē "retribution, penalty, quit-money for spilled blood," from PIE *kwei- "to pay, atone, compensate" (see penal).
The early "punishment" sense in English survives in phrase on pain of death. Also c. 1300 the word was used for the torments of eternal damnation after death. The sense of "exertion, effort" is from late 14c.; pains "great care taken (for some purpose), exertion or trouble taken in doing something" is recorded from 1520s.
Phrase give (someone) a pain "be annoying and irritating" is by 1895; as a noun, localized as pain in the neck (1924) and pain in the ass (1934), though this last might have gone long unrecorded and be the original sense and the others euphemisms. First record of pain-killer "drug or herb that reduces pain" is by 1845.
c. 1300, peinen, "to exert or strain oneself, strive; endeavor," from Old French pener (v.) "to hurt, cause pain," from peine, and from Middle English peine (n.); see pain (n.). Transitive meaning "cause pain; inflict pain" is from late 14c. That of "to cause sorrow, grief, or unhappiness" also is from late 14c. In Middle English also "to punish for an offense or fault; to torture, to torment." Related: Pained; paining.
"having power to relieve pain," 1540s, from Medieval Latin anodynus "pain-removing, allaying pain," from Latin anodynus "painless," from Greek anodynos "free from pain," from an- "without" (see an- (1)) + odyne "pain, torment" (of the body or mind), a word of uncertain origin, evidently Indo-European, but none of the proposed etymologies satisfies Beekes. Some suggest it is a suffixed form of PIE root *ed- "to eat" (compare Lithuanian ėdžioti "to devour, bite," ėdžiotis "to suffer pain").
As a noun, "substance which alleviates pain," 1540s; in old slang, frequently a euphemism for "death" (as the final relief from the mental pain or distress of life) as in anodyne necklace "hangman's noose." Related: Anodynous.
unit of intensity of pain, 1947, from Latin dolor "pain, grief, sorrow," perhaps from PIE root *delh- "to chop" "under the assumption than 'pain' was expressed by the feeling of 'being torn apart'" [de Vaan].