Flother is another word for a snowflake. It appears only once, in a 1275 CE book. The poison that killed Hamlet’s father in Shakespeare’s play, hebenon, is mentioned nowhere else. These are the hapax legomena, the lonely words.
The hapax legomenon is, essentially, a word that appears only once in a given corpus. For example, Shakespeare only uses the word satyr once (again, in Hamlet). Chaucer only uses nortelrye once – and, except for people writing about Chaucer, it doesn’t seem to have been used anywhere else.
It’s not just in English: Homer’s Iliad purportedly has more than a thousand. The Hebrew Bible has around four hundred, one of which is Lilith. Several Chinese characters only occur once in history.
In cases where a word only appears once in a whole language’s corpus, we cannot hope to discover their meaning without contextual clues or contemporary accounts. They’re essentially an island of language, separate from the mainland of meaning forever.
Also: don’t say any of these words out loud, or you may accidentally summon Cthulhu or something.