The following is my interpretation as a lay entymologist. I find it very likely to be correct, but it should not be taken as scholarly evidence.
The root word in the name Tuftin is the word «tuft», which means a lot or plot (of land), foundations for a building or a place with the remains of old buildings. Placenames with this root is found all over the country, and there are many surnames derived from such places. «tuftin» is most likely a dialect plural of tuft and it seems likely these places were named due to remnants of past occupation. There are just over a dozen places named Tuftin in the database of the Norwegian Mapping Authority, but to my knowledge the surname originates with just one of those, namely Tuftin in Sokna.
Tuftin started out as a small husmannsplass (a tenant farm or croft) belonging to the farm Røsby in Sokna, became a separatly owned property in 1918, and is today a vacation property owned by a descendant of the buyer. It has a view over the Sogna river and lower properties, being situated a ways up the Tuftin hill, Tuftinåsen. You can zoom out the embedded map to get an idea about where in Norway this is, zoom in to see where Tuftinåsen is, or open a full sized map.
Tenancy of farms like Tuftin rarely passed from father to son, so my genealogical interest in the history of the place is limited, but according to a local historian it is mentioned in a 1733 document describing an exchange of property including Nordre (Northern) Røsby, Tuftin and another tenant farm under Røsby.
In the Norwegian censuses of 1865, 1875 and 1900 Tuftin is listed as two farms subtitled nordre (northern) and søndre (southern). In 1910 it was one tenant farm. And, as mentioned before, it became a separately owned property in 1918. Information in the '65 and '75 censuses, the 1948 Buskerud volumes of Norwegian Farmsteads and modern property lines indicate a total size of Tuftin of 9.5 hectare (23 acres)*, with about a tenth of that being cultivated (1 hectare / 2 acres), the rest being forest and grazing. The two parts appear to have been roughly the same size, with søndre being the smaller.
Of the 23 people in Norway today (2011) with the surname Tuftin, a handfull are descendants of Petter Tuftin who had the tenancy of Tuftin in 1900 and 1910. I've researched the ancestors of Petter and his wife in the 19th century and found no link to my own, so they are only mentioned here since we share the name.
The remaining 18 are all descended (by adoption in one case) from Torsten Knutsen who was the tenant on Tuftin søndre from at least 1823 until his death in 1871. As described in more detail in my text about Norwegian naming traditions before the 20th century Torsten himself may have been called Torsten Tuftin at times, but it was not an inheritable surname as we think of it, more of an address or nickname. Still it must have been a name many of his descendants identified with, since it starts showing up in records for descendants two generations removed from the place itself.
Torsten's son Knut likely used the name, although there are no records showing it. The evidence lies in its use by his descendants. His son Gudbrand certainly used it. Gudbrand is the ancestor of the largest number of Norwegian and all of the Canadian and American Tuftins. One of Gudbrand's nephews, Torvald, also starts appearing in records as Torvald Tuftin in the early 20th century, and another nephew, Edward, used it after his emigration to America, although he changed the spelling to Thuftin.
Torsten's son Anders and his son Johan are mentioned in newspaper articles about bear hunting in the late 19th century with the surname Tuftin, but it later got shortened to Tuft which was the surname used by descendants of both Johan and his brother Anton. Anders' daughter Anette however lived at times at Tuftin and has descendants using that name today.
Torsten's youngest son, Ole, emigrated to Sweden where he used the surname Tufting. Only his daughters survived to adulthood so the name is not in use today.
*The size of Tuftin
Figuring out the size of the Tuftin farm(s) requires a little conjecture, but lets start with the bare facts. I've based my calculation on the following sources.
Helge Ellingsen 76, 1 horse, 2 heads of cattle, 3 sheep, 1 barrel of mixed grain, 2 barrels of potatoes
Torsten Knutsen 79, 1 head of cattle, 2 sheep, 1/4 barrel of mixed grain, 1/2 barrel of potatoes
Tuftin (nordre), Helge Ellingsen 85, 2 heads of cattle, 4 sheep, 3/8 barrel of barley, 1 barrel of mixed grain, 2 barrels of potatoes, 1/32 maal (1 maal abt. 1000 m2) other root vegetables
Tuftin (søndre), Ole Bjørnsen 27, 1 head of cattle, 2 sheep, 1/4 barrel of barley, 3/4 barrels of mixed grain, 4 pounds of grass seed, 2 barrels of potatoes, 1/32 maal (1 maal abt. 1000 m2) other root vegetables
Petter Tuftin 48, size not given but a tenant farm with plowed land, cattle, poultry and a vegetable garden
Isak Strømsodt 65, size not given but a croft with plowed land, cattle and poultry
Petter Andersen Tuftin 58, size not given but still a tenant farm
1948 Norwegian Farmsteads
Johan Solberg 65, submits that he bought the farm in 1918, has cleared 0.8 hectare to make the total farmland 1.8 hectare cultivated, 1.8 hectare other farmland and 4 hectare forest and grazing. 1 horse, 3 heads of cattle, 1 calf, 2 pigs, 3 sheep, 10 chickens. Picture.
Present day property lines
Tuftin, leisure property. abt. 97 000 m2 using the Norwegian Mapping Authority's online map tools.
The 19th century barrel could be a measure of area equal to 3 937 m², but for a couple of reasons it seems more likely to me the censuses recorded the actual volume of seed used. Using statistical data from the late 19th century for the volume of seed per area I get a cultivated area in 1875 of 1.3 hectare, whereas the barrel as an measure of area give a cultivated area for all of Tuftin of less than half a hectare. Considering the 1948 statement the farm originally had 1 hectare cultivated land I find the 1.3 figure more likely. I also believe actual use of seed to be an easier statistic to collect than the area used.
There's a discrepancy between the 1948 numbers and the property today of nearly 2 hectare. I find it likely that the present day property lines are roughly those of the 19th century tenancies and that the discrepancy is due to how the numbers were reported in 1948.