Rats! And I just saw this term recently in this very forum, but I must not be searching correctly. It the term for when you have commonly understood words that over time lose a syllable (usually the prefix).
Sort-of-not-really example: if the word “inflammable” completely vanished and all we ever used was “flammable”.
Fake example with made up word: “bosmutpurdoodle” becomes “smutpurdoodle” and several generations have never heard of the original word.