One of the ironies of my own research is that the great-grandparent who I knew the best is the one whose ancestry I know the least about. Suzanne Jeanne Gardahaut was born on Nov. 9, 1900 in Nantes, France and lived actively into her nineties. She was my one connection to something exotic in my own family background. Not only was she French, but she was something more, she was a Breton, from the culturally distinct region of Brittany in northwest France. This fascinated me, largely because it was something that made me feel unique. When it came time to make a display for French night, I proudly did it on Brittany. When I had the chance, I even took a course on Breton, the wonderful Celtic language of Brittany, which is related to Welsh and Cornish.
Along with this unusual heritage (at least where I lived), she also had an unusual surname. The name is absent from US Census (my great-grandmother married soon after she immigrated and is listed under her husband's name). The only historical record for the name on FamilySearch.org is my great-grandmother's ship record. The only place left to look is in France. One useful tool is the La France de noms on the genealogie.com website which can give you an idea of the distribution of surnames in France. The Gardahauts, few that there are, are clustered in or near Brittany in the northwest corner of France on the map for the period 1891-1915.
The meaning of the names is fairly obscure. One theory put forward on the GeneaNet (here) holds that the name is from the phrase garde à eau referring to a reservoir or mill race or is related to a place name Gardehaut in Côtes d'Armor. The evidence seems mainly speculative in either case.