- Motto: "Fnargh."
I'm going to cheat (:P) and use my Uni access to the OED Online, which I recommend for any kind of etymological need!
Dough - Etymology: A Common Germanic n.: Old English dáh, genitive dáges, = Old Frisian deeg, Dutch deg, Old High German, Middle High German teic, German teig, Old Norse deig, (Swedish deg, Danish deig, dei), Gothic daigs < Old Germanic *daigoz, < verbal stem dig-, deig-, pre-Germanic *dhigh- to form of clay, to knead: compare Sanskrit dih- to besmear, Latin fig-, fingĕre; compare Greek τεῖχος wall.
Tough - Etymology: Old English tóh < *tǫnh < *tanh, Old Germanic *taŋχu-z; North Frisian toch, tuch. From an Old Germanic stem *taŋχ-, taŋg-, whence Old English ge-tęnge. Compare (with ending of -ja declension) Old Saxon *tâhi (Middle Low German tâ, tei, Low German taa, tage, tau, Dutch taai); Old High German zâhi (Middle Low German zâhe, zæhe, zæch, German zähe, zäh)
The pronunciation for tough seems to remain pretty much unvaried, and looking very close to present-day glottals in Dutch 'g' and Scots 'ch'. The switch to a softer sound appears as of the 17th century in Middle English.
As for dough, again it's arounf C17 that the sound changes to the almost invisible present-day pronunciation. Similar to 'way' and 'day', tough. There is still a U.S. and British regional version of 'duff', meaning backside, which relates back to 'dough'.