Meaning and information about Nessie, What does Nessie mean? Nessie means Diminutive of Agnes: Poor, pure, or chaste. St. Agnes was a 3rd century Christian martyr whose January 21st feast day is described in Keats' poem 'The Eve of St Agnes'. Agnes was popular until the Reformation, then revived in the 19th century.Pure. Find similar names like Nessie.