How does the word "thunder" get the letter "d"?

Question

thunder

O.E. þunor, from P.Gmc. thunraz (cf. O.N. þorr, O.Fris. thuner, M.Du. donre, Du. donder, O.H.G. donar, Ger. Donner "thunder"), from PIE (s)tene- "to resound, thunder" (cf. Skt. tanayitnuh "thundering," Pers. tundar "thunder," L. tonare "to thunder"). Swed. tordön is lit. "Thor's din." The intrusive -d- is also found in Dutch and Icelandic versions of the word.

The "d" in Swedish "tordön" comes from "Thor's din", and the question arises, namely whether the common "d" appears in English "thunder", Dutch "donder", and Icelandic word has the same origin as the Swedish word?

Answer 1

Campbell (2004: 36) gives the OE form of 'thunder' as þunrian (though whether this OE form or the other is considered shouldn't affect the explanation), and indicates the addition of the stop consonant to be an example of excrescence, along with þy:mel > thimble.

Note that in both examples the excrescent stop follows a nasal consonant and precedes an oral sonorant. The transition from a nasal consonant to an oral sonorant requires a velum-raising gesture and an obstruction-releasing gesture to happen more or less at the same time. The production of a voiced stop following a nasal consonant happens naturally and spontaneously if the obstruction-releasing gesture lags the velum-lowering gesture just a little bit.

It may also be of anecdotal interest to note that in a number of languages (e.g., Korean, some Amazonian languages) what are described as nasal consonants may be pronounced as either nasals, voiced stops or prenasalized voiced stops.

Campbell, L. (1998/2004). Historical Linguistics: An Introduction. MIT Press.

Answer 2

I can suggest two reasons: The 'd' could have been added under influence from the Norse forms during the Viking age (when English adopted significant quantities of Norse vocabulary and grammar).

Alternatively, it may have been added as a hypercorrection - it's a common tendency in English dialects today to drop the alveolar stops t and d, especially after 'n' ('Toronto' is often pronounced 'Tronna' by residents, for example). The 'd' might have been added by people who wanted to sound more learned, and mistakenly thought they were dropping a 'd' in that word.