Is "Many thanks" a proper usage?
Solution 1:
Yes, many thanks is perfectly proper, grammatical, standard English. It is appropriate to use wherever "thanks" (as opposed to "thank you") would be acceptable.
Solution 2:
As Martha says, many thanks is perfectly idiomatic. However, it is indeed an oddly isolated idiom: most other constructions which try to treat thanks as a plural noun are ungrammatical (eg *lots of thanks), and there’s certainly no such thing as *a thank.
In the sense of “feelings of gratitude” it can be used either as a mass noun (thanks is due to God for this harvest…), or a plural (thanks are due to God…). In the sense of “expressions of gratitude”, it can be used as either of these, or also as a singular: a special thanks is due to the chairman…
[Sources for this: roughly counting hits for phrasings like thanks is, thanks are in Google and COHA.]
How did it get this way? The entries for thanks in the OED tell a tangled tale, but as far as I can make out, thank was originally an acceptable singular noun (their last citation is 1642, Is this the thanke which you returne to God?), but the plural form thanks gradually predominated, and apparently lost its plural-ness to some extent.
This bit is confusing: the OED describes thanks here as “†Formerly sometimes construed as singular”, and their citations where it’s unambiguously singular stop after Shakespeare (1594, Thanks to men Of Noble minds, is honourable meede.), but all the later citations they give are completely ambiguous about its number (1805, I return it to you with my sincere thanks.), like most modern usage. There are no citations where it’s unambiguously treated as plural.
Their earliest citation for the phrase Many thanks is 1803, Many thanks for your letter. So this seems to arise some while after thanks was generally construed as plural! I’d love to know more of the background here…
Solution 3:
It is heard several times every morning on BBC Radio 4's Today programme: it is the standard closing that they use after interviewing somebody.