Northrup, A. Judd (Ansel Judd), 1833-1919. Northrup family (Joseph Northrup, d. 1669). New York : Grafton Press. (Not in copyright)
http://www.archive.org/details/northrupnorthrop00nort
Some effort has been made to find the name in England, to determine its origin and proper spelling. Seven different forms have been found. Maude, daughter of Simon Northrope, married in County York, in the reign of Henry VII (1485-1509). Elizabeth Northrope married April 21, 1669. William Northrope is mentioned in will, Dec. 20, 1485 ; Henry Northrope, 1489-90, in Beverly. Wills in York Registry give names of Robert Northrope, 1591 ; Richard Northrope, 1611; George Northrope, of Heaton, 1618; Wilham Northrope, 1616; John Northrope, 1592; William del Northrope, 1288-1294, on jury. All these names terminate in -rope. No other forms appear in the York County records as far as examined. These forms also appear elsewhere in England : Northorp, Northorp, Northopp, Northrope, Northrop, Northropp, del Northrope.
In the published official list of soldiers of the Revolution in Massachusetts, the following forms of the name appear : Northrup, Northrupt, Northrop, Northorp, Northroup, Nortrop, Northrip. In the Yorkshire Parish Register, Vol. IX, appear the following: 1604, Northrop, Northrope; 1617, Northrope; 1649, Northrope; 1664, Northroppe. The Hertfordshire records contain the names Northrope, Northroppe, Northropp (1599). Northorp appears in the Parish Registry of Horbury.
These facts show clearly that at least in York County and Hertfordshire the name, for a long period, has been Northrop, or Northrope. The other forms, -thorp, -op, -oppe, are evidently modifications of Northrop.
The name is a compound of North and the Saxon word thorp (appearing in such names as Althorp) and the Middle English thrope, meaning town or village. John of North-Thorpe became "John Northorpe." This was naturally changed, for ease of speaking it, to Northrope, or Northrop.
Winthrop and Lathrop are examples of the same change. The change to Northrup may have been a change to correspond to a manner of speaking the word "Northrop," or, in some parts of England the name in that form may have been common, or the change may have been arbitrarily made. I think, however, the original name in England — certainly in Yorkshire and Hertfordshire — was Northorpe, and then "Northrop." Whether this form ever prevailed in some parts of England, or was an arbitrary change made by Joseph, I have no opinion.
Joseph Northrup, the immigrant, called himself Northrop. In the early records and deeds of Milford he spelled his name Northrup. Land grants to him were to Joseph Northrop (Town Clerk's letter). In Barber's history of Connecticut he spells the name of Joseph Northrup. His son Joseph's tombstone bears the same name, and his sons, Moses, James and Joseph, who went to Ridgefield, and most of their descendants, spelled their names Northrup. This Moses made a will (which I have seen), in which the scribe who drew it gave his name as "Northrop," but Moses signed it "Northrup." I have no means of determining how the other sons of the first Joseph spelled their names (except that Samuel's family is said to have spelled the name -rup), nor how or how soon the form Northrop came into common use.