on/under pain of something

on pain of

With the threat of undergoing something extremely bad or unpleasant as a punishment. Often used in the phrase "on pain of death." The prisoners were forced to march on pain of death from the POW camp across the burning desert. On pain of having our pay docked, we were told we had to come in every Saturday until further notice to take care of the backlog of work.
See also: of, on, pain

under pain of

With the threat of undergoing something extremely bad or unpleasant as a punishment. (Often used in the phrase "under pain of death.") Under pain of having our pay docked, we were told we had to come in every Saturday until further notice to take care of the backlog in work. The prisoners were forced to march under pain of death from the POW camp across the burning desert.
See also: of, pain

on pain of

Also, under pain of. Subject to the penalty of a specific punishment. For example, The air traffic controllers knew that going on strike was on pain of losing their jobs . At one time this idiom often invoked death as the penalty, a usage that is largely hyperbolic today, as in We'd better be back on time, under pain of death. [Late 1300s]
See also: of, on, pain

on/under pain of something

(formal) with the threat of having something done to you as a punishment if you do not obey a command: They were forbidden on pain of death to talk to any of the other prisoners.We were told to pay within three days, on pain of a €1 000 fine if we didn’t.
See also: of, on, pain, something