the.com/fossilword
a word that survives only inside one fixed phrase, extinct everywhere else in the language.
means a term that has died out of general use but persists frozen inside a single idiom or expression.
from linguists borrowed the geology metaphor: like a fossil trapped in rock, the word is preserved not because it thrives but because the surrounding phrase hardened around it before it could go extinct.
kith survivesonly in kith and kin, otherwise dead since 1900s
vim examplelives solely inside vim and vigor
academic termlinguists call these fossils or relic forms
cranberry cousinrelated to cranberry morphemes like the cran in cranberry
for instance
kith and kin — kith meaning countrymen has no other modern use
to and fro — fro survives nowhere except this pairing
spick and span — spick is a dead word for nail or spike
rank and file — file here is a fossil military sense of row