Lexicalization

Lexicalization is phenomenon by which a morphologically complex word starts to behave like an underived word in some respect, which means that at least one feature (semantic, syntactic, or phonological) becomes unpredictable. Thus a lexicalized word has at least one aspect which cannot be predicted by the general rules of grammar.

Example
the Dutch adjective reusachtig has two readings, one is transparently derived from the noun reus_N and the suffix -achtig which can be paraphrased as 'like a reus (giant)', and one which simply means 'very big'. This latter reading is lexicalized. Additional evidence for this comes from the fact that in this latter reading the pronunciation is slightly different: the final phoneme of reus is pronounced as [z], while it is pronounced as [s] in the transparent reading. Assuming that lexicalization eradicates internal boundaries, the phonological rule FINAL DEVOICING cannot apply in the lexicalized form.

Link
Utrecht Lexicon of Linguistics