The British spelling: to turn into a fossil or to preserve something unchanged for a long time.
From fossil (Latin fossilis 'dug up') + -ise (British variant of -ize, from Greek -izein). British English retained the 's' while American English shifted to 'z' around the 1800s.
The phrase 'fossilised ideas' is used metaphorically to mean old-fashioned thinking that won't change—showing how a word about geology became a powerful way to criticize rigid thinking!
Complete word intelligence in one call. Free tier — 50 lookups/day.