Latin 'sub poena,' under penalty — the writ named after its threat: comply or face the stated punishment.
A writ ordering a person to attend a court and give testimony or produce documents, under penalty for failure to comply; the legal process compelling attendance or disclosure.
From Medieval Latin 'sub poena,' meaning 'under penalty' — the opening words of the writ. 'Sub' (under, beneath) comes from PIE *upo (under, up from under). 'Poena' (punishment, penalty) was borrowed into Latin from Greek 'poinē' (blood-money, penalty, retribution), from PIE *kʷey- (to pay, to atone).
The words 'pain,' 'penalty,' 'punish,' and 'impunity' all descend from Latin 'poena,' which itself came from Greek 'poinē' — blood-money paid to compensate a murder victim's family. The concept of legal penalty thus began as a payment to buy peace after a killing, and the word for pain in English traces back to that same ancient system of compensation.