Keller's Learn to Read Latin says:
The adjective Idem, eadem, idem is formed by the addition of the suffix -dem to the demonstrative adjective is, ea, id.
What does suffix -dem mean? I couldn't find it in the Oxford Latin Dictionary.
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In practice it means nothing. What I mean by this is that the suffix -dem has only very special uses, you cannot use it productively on new words, and it does not have an easily predictable effect on the meaning. It is therefore practical not to treat -dem as a suffix, apart from remembering how to inflect the pronoun idem. It therefore makes perfect sense for the suffix not to appear in dictionaries.
The etymology of the suffix -dem and whether it is a misinterpretation of an actual suffix -em as mentioned by Wiktionary I will leave to others. That is interesting, but not very relevant for learning to use Latin.