The Latin verb mandare, meaning "to entrust" or "to order," is the authority behind countermand. It's also behind the words mandate, command, demand, commend (which can mean "to entrust" as well as "to praise"), and mandatory. Countermand came to English via Anglo French, where the prefix cuntre- ("against") was combined with the verb mander ("to command"). It has been a part of English since the 1400s.