The name "Plath" derives from the German word "platt," meaning "flat and wide," as is the topography of northern coastal Germany and the Netherlands: boggy and marshy coastal plain.
Sylvia's father Otto Plath was born about 300 miles away in the Prussian town of Grabow, now in Poland. (See the map.) Otto and his five siblings grew up in his father's hometown of Budszyn, formerly in Prussia, now in Poland, about 200 miles from the town of Plath, so it has been a while since Sylvia's paternal line actually lived in Plath, if ever. Slavic tribes in the 700s conquered the area's original Germanic residents, lived there 400 years, then were Christianized and Germanized, so bloodlines in northeastern Germany are not pure Germanic but mixed.
The language "Plattdeutsch," in English called Low German, akin to Dutch, is a variation of the Saxon language and therefore an ancestor of English. When schools teach German, it is
the more common "Hochdeutsch" or High German. (This is a geographical reference to "highlands" and not a
value judgement.)