The origin of this Interesting and puzzling surname Thayer could be English, variants of the name Thayer include Theyer, Thyer, Thair, and Thayre. There is also bearers from Germany who has the surname Thayer, pointing of a German origin.
The origin of the name Thayer is an interesting one, but uncertained. The name Thayer could origin from the following. First (and I think most likely), the surname Thayer is an English form of french word "théière", meaning "tea-pot". Perhaps the surname Thayer was given to those who made tea pots or something to do with tea. Close variants surnames include Theier, Thaier, and Theyere, very close spellings to french Theiere, and also is pronounce almost identical to Theyer. The Thayer/Theyers pronounced their surname as "Tayer and Teyer", which same goes to french word Theiere. The English surname Thayer is mostly found in counties Sussex, Gloucestershire, and Somerset. Perhaps the Thayers/Theyers from Gloucestershire and Somerset, and the Thayers/Thairs/Thayres from Sussex are not related, and that the origins of both Thayers are different. A second possible origin is that the name Thayer is a shorten form of the German name "Theudoric", "theudo" meaning "people, race", and "ric" meaning "power", or perhaps shorten form of German name "Theudhard", "hard" meaning "brave, strong", which I think Theudhard seems more likely becuase of its close spelling and sound. Similarly, Will shortened from William, Bob shortened from Robert, and perhaps Thayer shortened from Theudhard or Theudoric, same as Terry. The first name Thayer is also common. The surname Thayer and other variants are mostly found in Southern Germany and mostly in Austria. The German surname Thier is also mostly found in Austria and Southern Germany, meaning "wild animal", perhaps a name for a hunter. This probably explains the Thayer families from Germany and Austria, a short form of Theudhard or Theudoric, or a variant of Thier. The surname Thier is also found in France, perhaps there is of French origin. Perhaps there is a French surname meaning only "tea" and the Thayers/Theyers could derived from that also.
Another possible origin is that the name Thayer derived from English "tawyer": one who dressed skins. Surnames of this include Tayer, Tawyer, and Tawer, but these surnames are extremely rare.
As you have said already Alan, I dont think the name Thayer derived from French word Taillier, also as English suranme Taylor and Tailor, but perhaps this applies to a few Thayer families out there...
More information is here - http://www.thayerfamilies.com/http://www.angelfire.com/super/rugbyprincess/Family_Hertiage_Paper.htmhttp://archiver.rootsweb.ancestry.com/th/read/THAYER/2002-09/1033220011http://49.americanancestors.org/two-kinds-of-thayers/http://www.ancestry.com/facts/Thayer-family-history-uk.ashx