The earliest known reference to the Armstrongs having descended from Earl Siward of Northumberland comes from some 18th century correspondence between two Irish Parsons.
The great earl's grandson, Siward Fairbeorn the White is said to have been among the many refugees fleeing to Scotland after the Norman conquest of England. King Malcom III met Siward with great kindness, and they fought the Normans together. In one battle Malcom's horse was slain under him, and he himself wounded. Young Siward fought his way to the king's side and, passing his left arm around his monarch's body, cut his way through the enemy to a place of safety. (Another version states that Siward gave Malcom his horse.) For his courageous act, Siward Fairbeorn was knighted, given land and a castle on the Scottish border, and from that time on was referred to as the Sword of the Strong Arm (or Armstrong). This Siward was said to be ancestor to the lands of Mangerton in Liddesdale.
J. Alan Armstrong, of the Clan Armstrong Trust, believes these stories are incorrect. He writes "Much of what you find in the Chronicles of the Armstrongs, regarding Siward, are basically correct but as for our descent from this man I can assure you that it is totally incorrect... If you examine the dates of Siward and the date of the second Laird of Mangerton, one lived in the llth century and the next in the 14th century. A three hundred year gap! The earliest reference to anyone using the (Armstrong) name in historical documents is in the 13th century and appears in a charter relating to a monastic settlement in Cumberland. The earliest reference to the Armstrongs in Scotland appears in a land rent roll of the 14th century... There is a connection with Siward but it is very tenuous and only arises through a succession of female marriages and linked with the Bruces."