Saint Cyneburh (d. c. 680) was the Anglo-Saxon abbess of Castor in Northamptonshire — daughter of King Penda of Mercia and sister of Saint Cuthberga. She married Alhfrith of Deira before becoming abbess.
Featured in Bede's Ecclesiastical History.
Cyneburh does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Cyneburh reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.