Calenardhon was a wide green landscape in the north of Gondor, but it became depopulated during the Third Age. It was devastated by the Great Plague of Third Age 1636 but some Dunlendings settled some of its lands.
During the rule of the Steward Cirion, Gondor faced a series of attacks by the Balchoth, and Calenardhon was overrun by these evil people. Cirion sent messages for help to Gondor's old allies, including the Éothéod, to help defend Gondor.
The hypothetical Old Sindarin would likely be *Kalinarda.