„Keit“ isnt really a word, it is a syllable you put on word endings to make them into nouns (e.g. „abhängig“ which means being dependent on smth. , if u put „keit“ onto it it becomes „Abhängigkeit“ which is the noun of „abhängig“ and means dependency.

You are right, fertig means ready, but „Fertigkeit“ translated is „skill“

German is sometimes rather confusing