There are many words in Germanic languages whose roots are difficult to identify. Some believe that the lack of clear cognates among other Indo-European languages is indicative of a mixed origin for the Germanic languages.

One group of these words has to do with ships and the sea; words like keel, oar, and steer are shared by almost every Germanic language (or at least by the Scandinavian ones), but cognates for these specific words and senses are not found in other branches of Indo-European. This likely reflects the land-locked nature of the Indo-European homeland. Another group of these words deals with war and weapons; words like sword, shield, and bow are all found in almost every Germanic language, but again, not with these meanings among other Indo-European languages (knight is in this class as well, but does not usually have a military meaning). Some names for animals such as eel and dog are also among these words of obscure origin; so are a few farm animals like calf. There are scores of non-Indo-European words that are used daily by English speakers; words like earth, blood, hand, wife, evil, little, sick, bring, and house. Among the Germanic languages, these words are found everywhere, north, south, and west (but not east, which is cognate to the Greek Auos, "dawn", as well as Easter); outside the Germanic family, cognates are unknown, or have been borrowed from Germanic. A few of these words may be shared with the Celtic languages, but otherwise no other language has them. Etymologies have been proposed for some of these words that link them with Indo-European roots, but these etymologies have no parallels in other Indo-European families in either form or meaning.

A number of theories have been advanced about this hypothetical cultural and linguistic substrate. Some refer to these unknown people as Folkish, because they believe folk to be a word of non-Indo-European origin; others refer to them as the Battle-axe people. If this substrate did exist, it cannot be conclusively identified with any pre-historic or historic European language or culture.

It should, however, be noted that historical linguistics is a very hit-and-miss affair. Even words known to have very recent origins often have unknown or uncertain etymologies, and the etymology of long established words may often be no more than guesswork. It is unreasonable to expect to be able to precisely identify the origins of every word in a language with complete certainty. Historical linguistics works with a number of words at once, and tries to decide if they have a systematic phonetic relationship to words in another language. Thus, any hypothesis about an unknown substrate language is necessarily speculative when there is no possibility of comparing two or more languages.

However, there has in recent years been a revival of interest in contact linguistics and the idea that intercultural contact may be the major force behind linguistic change has recently become quite mainstream. This could ultimately lead to a renewal of interest in early and pre-Indo-European Europe.