I'm interested in the name of the concept that defines the word from which another word comes. For example, "Guild" comes from the German "Gilde". What is the name of the word "Gilde" in this case? Original, first or something else? I tried to google, ask ChatGPT, but so far I haven’t found anything adequate. It seems to say that this is called “proto-form”, but this word refers to chemistry, as I understand it
Proto-form may also be used in chemistry, but in linguistics it refers to a reconstructed form from which other forms are derived (see proto-language and comparative method). An example of a proto-form is Proto-Indo-European *gheh1bh 'to grab, take', from which we get Latin habēo, habitus > English habit, but also Welsh gafael 'to hold, grasp'. Like the "proto" in "proto-form", the "proto" in "Proto-Indo-European" indicates that this language is not actually attested in any sources but that it is reconstructed on the basis of evidence from languages that are related to each other.
In "*gheh1bh", the asterisk indicates that this is form is not actually attested but is a reconstructed proto-form. Not all proto-forms have lots of superscripts and subscripts; this depends on the complexity of the reconstructed phonology. The Hebrew and Arabic for 'son' can be reconstructed as Proto-Semitic *bin-, for example.
"Proto-form" may be what you are looking for, though it is not strictly speaking a "known word", since we do not have evidence for it. In the case of habit above, Latin habēo appears to be the first attested form from which English habit is derived.
In the case of guild, Wiktionary derives it from Old Norse gildi rather than German Gilde (though the latter is of course related, and Wiktionary may be wrong, too). This is a nominal derivation of the verb gjalda 'to pay', which is reconstructed for Proto-Germanic as *geldaną. So here the first attested word from which guild is derived could be said to be gjalda, though you could also decide to exclude morphological derivations, in which case it would be gildi.
I am not aware of a specific term for this concept, but "first attested form" seems to work alright. You should be aware, however, that guild may not actually be derived from any of the attested forms of gildi. For example, gildi may have been loaned into English in the year X, while gildi only appears in Old Norse sources in X+50. In that case, gildi is the source for guild, but none of the actually attested instances of gildi are the source for guild. Does gildi still count as the "first attested form" of guild?
I don't believe there is a single word. I would describe this as the "earliest attested antecedent". I might also use another word like "predecessor", "etymon", or "form" instead of "antecedent", depending on the specific example and relationship between it and the current word under consideration (in particular I would be less likely to use the word "form" if the word has been borrowed, as in this case). The key part here is "earliest attested".
Proto-form would imply to me that the term is a reconstruction. I would not use it for an attested word.