Is there a word we lost the definition to? A word whose definition we lost to history? Something that is a part of our history but we forgot the meaning with time
Ancient Greek word ΣΑΣΤΗΡ (sastēr)
From 1890 to 1899, in pieces, a white marble slab was found by archaeologists in the ruins of an Ancient Greek colony Chersonesus, Greek Χερσόνησος (Khersónēsos), on the Crimean Peninsula, established in the 6th century BC. The slab (photo) was inscribed with a text in Ancient Greek being the civic oath of the Chersonesites (citizens of Chersonesus): description, Ancient Greek text and English translation. The text is thought to be inscribed in the beginning of the 3rd century BC.
Among the understandable oaths (“I will not betray anything to anyone, neither a Hellenic nor a barbarian,” “I will not violate democracy,” “I will not plot a conspiracy,” “I will be an enemy to malefactors”) there is one: “I will protect the saster (ΣΑΣΤΗΡ) for the people" (και τον σαστηρα τωι δαμωι διαφυλαξω, lines 24–25).
This word is not found in any other Greek text of the Old or New time. The literature on saster is extensive. There are many hypotheses, including some very eccentric ones. Max Fasmer and Lev Yelnitsky, for example, believed that the saster was the Scythian governor of Chersonesos, S. A. Zhebelev — that it was some kind of sacred object, for example an idol; V.V. Latyshev (the first publisher of the inscription) - that this is a kind of legal concept, for example, a civil oath. Most recently, I. Markov argues it is the city treasury. Parallels were sought for this word in Iranian and other languages. Historical novels appeared, which featured the sacred saster towering over the Chersonesus coast; in Sevastopol, a modern city situated near the ancient Chersonesus, a festival called "Saster of Chersonesos" was held. On the Internet, you can listen to a song with the words "And I will find a magic saster" (with an emphasis on "a") and read verses with the line "An unknown saster hiding from us" (with an emphasis on "e").
No one knows for sure what ΣΑΣΤΗΡ is.
There are many such words. Even for a really well-attested dead language like Latin such words are known, e.g., aurichalc, haematopus, or cortumio (all three examples taken from the answers to this question on latin.se)
A surprising example is that one of the words in the "Lord's Prayer", one of the most significant prayers of the Christian tradition, has an unknown meaning.
The original Greek word is epiousios (ἐπιούσιος) and has traditionally been translated as "daily" - but that translation has no particularly strong foundation. It occurs in the phrase "Τὸν ἄρτον ἡμῶν τὸν ἐπιούσιον δὸς ἡμῖν σήμερον", or "Give us today our epiousion bread".
See for example the Wikipedia page for epiousios.
In Genesis 6:14, Noah's Ark is made of עצי גפר (gopher wood). "Gopher" is just a phonetic transliteration of the ancient Hebrew גֹּפֶר. No one knows what it means, except that it is presumably some kind of wood.
This is defined as a word that only appears once in a given context - it can be in a single book, an author's complete works, or in the published works of an entire language (whether a dead language or an extant one). In the last sense, it would generally be a word whose definition is lost to history. The linked article gives a few examples.
(Note: This is not a lost word itself, but a word that potentially describes such words.)
The bible is full of such words. We can suspect what they mean (and being the bible - there are "canon" meanings). But we don't really know. Many of these words were migrated to modern Hebrew - and got a modern unrelated meaning.
for example - אקדח (gun). We know that it was probably some kind of gem. But we don't know what kind. https://www.studylight.org/lexicons/eng/hebrew/688.html