When used by Jehovah’s Witnesses:

Short for “disfellowshipped” (see disfellowshipping).

Frequently used as a code word because the speaker does not want non-Jehovah’s Witnesses to understand what they are talking about. This reflects the fact that many JWs are ashamed of or embarasssed by the practice.

However, it is sometimes used purely out of laziness as the word “disfellowshipped” is very long.
Jeremy can’t come with us to Europe anymore because he got DF’ed.
by JW Terms October 20, 2018
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