(statistics) Initialism of inverse document frequency.

(military) Initialism of Iceland Defense Force. / (military) Initialism of Irish Defence Forces. / (military) Initialism of Israel Defense Forces.

(information retrieval) A mathematical approximation to the importance of a particular word in a given piece of text.

(Internet slang, text messaging, vulgar) Initialism of I don't fucking know.

(Internet slang, text messaging, vulgar) Alternative letter-case form of IDFK.