Take F(x) to be any bijective map from the set of letters [a...z] to the set of integers [0...25]. A Fibnoacci-Word, as used here, is a word, WORD, such that [F(letter) for letter in WORD] is a fibonacci number.
So given the one-to-one map between the letters a...z and the numbers 0...25, the word can be expressed as a fibonacci number.
As an example, take the word "aphorismos".
If we use the map where:
a: 4
p: 8
h: 0
o: 7
r: 5
i: 2
s: 6
m: 9
Then we get that "aphorismos" can be represented as the 48th Fibonacci number, 4807526976.
This script finds the longest word that is also a fibonacci-word. Note, it requires some long list of words (presumeably a long list of valid English words), which I've not included in this repository. An example output is included in "output.txt"