Definition of "sirname"

Fair Helen's sirname, therefore, depends upon the period at which she lived, which it is now impossible to ascertain.

1806, Walter Scott, Minstrelsy of the Scottish Border, Vol. II (of 3)‎

These names were not always used; commonly two, and sometimes only the sirname.

1833, Charles K. Dillaway, Roman Antiquities, and Ancient Mythology‎

They were in two trunks (made exactly fit for them) that had the first letters of my christian and sirname decyphered upon them with nails; and on the first page of every book was my ordinary motto and name written at length in my own hand.

1902, William Younger Fletcher, English Book Collectors‎