I ACTUALLY KNOW EXACTLY HOW THIS HAPPENED.

When typing up the instructions, the pharmacist or technician was probably using shorthand in the computer, which automatically translates into text a patient can understand. The shorthand for this would have originally been ‘2 pfs po q4h prn hs’, where ‘prn’ stands for ‘as needed’. And there was a typo, so ‘prn’ became ‘pr’, which stands for ‘per rectum’ (aka rectally). I’ve done this myself on accident with ‘diabetes’ (DIAB) and ‘diarrhea’ (DIA), as well as prn and pr. 

…And that was your medical jargon lesson for the day, kiddies.