Source of word 'Ingvaeonic'?

The development of Ingvaeonic languages as a distinct group would have post dated Tacitus by several centuries. He did describe the people on the North Sea coast as Ingvaeonic (http://en.wikipedia.org/wiki/Ingaevones), and that area happens to be part of the area where Frisian is spoken today, hence the revival of the term in modern times to describe the commonalities specific to Frisian and English.

Low-Saxon and Dutch are also considered North Sea Germanic languages, but are not Ingvaeonic in the linguistic sense. Ingvaeonic has little meaning today. Afrikaans, Dutch (non Ingvaeonic) and Frisian (Ingvaeonic) are mutually intelligible, though Frisian and English are not. Much is made about the closeness between Frisian and English. The term Ingvaeonic from Tacitus' time was called upon to describe this group. Truth is, Frisian may be the closest language to English, but English is not the closest language to Frisian.

From an English perspective, Dutch is not signifficantly more distant than Frisian.

"Ingevaenic" and "North Sea Germanic" are used to describe which sound shifts these languages share. "North Sea Germanic" languages are West Germanic languages that does not share the German sound shift, from "s" to "t" at the end of words, nor the "k" to "ch". Hence Dutch maken, English make, German machen. Ingeveonic are languages in which share some changes, for example the "k" in some places shifted to "ch" (tj) sound. This affected Frisian and Old English, but not Dutch.

According to Wikipedia, "Ingvaeonic" is named after the "Ingaevones" which means "people of Yngvi".

And "Yngvi" (or Yngvin, or Ingwine, or Inguin) is an old Norse name which was derived from an older yet word that ultimately meant "worshiper or friend of Ing", and legend has Ingwax as the ancestor of the Ingaevones.

"Ingaevones" is described in Tacitus's Germania where a West Germanic cultural group or proto-tribe lived along the North Sea coast.