Linguistics Stack Exchange is a question and answer site for professional linguists and others with an interest in linguistic research and theory. It only takes a minute to sign up.
Sign up to join this community
The PIE *septm should have changed to Pre-PG *sefθen, by the Grimm's Law. Then, by the Verner's Law, it should have changed to *sebθen. Why did the *θ disappear for the word to develop into "sieben" in German and into "seven" ('b' regularly changes to 'v' in English between two vowels) in English?
I'd expect it to have developed into German **siebden ('th' changes to 'd' in German regularly) and English **sevthen or **sebthen.
The excellent German etymological dictionary by Pfeiffer has this:
sieben Num. Ahd. sibun (8. Jh.), mhd. siben (md. siven), asächs. siҍun, mnd. sēven, sȫven, mnl. sēven, nl. zeven, aengl. seofon, engl. seven, anord. sjau (wohl umgebildet durch frühen Einfluß des unter acht, s. d., behandelten Zahlworts, vgl. got. ahtau), schwed. sju, got. sibun (germ. *sebun) lassen sich mit aind. saptá, griech. heptá (ἑπτά), lat. septem, air. secht n- auf ie. *septṃ ‘sieben’ zurückführen, wobei das Germ. den inlautenden Dental aufgegeben hat, wohl dissimilatorisch unter Einfluß der gleichfalls mit Dental gebildeten Ordinalzahl ie. *sep(t)ṃto-.
In other words: back-formation from the ordinal, with dissimilation of t...t.
