Translation:Where is the plate that I was eating from?
Hmm, yes, "jejž" and "nějž" for genitive singular are missing from the wiktionary. The "Příručka" is the definitive source that has authority, it's issued by the Czech language institute, which dictates what's standard. And it includes these genitive singular forms for masc. anim., masc. inanim., and also neuter.
Apparently, these forms first appeared officially in a grammar book in 1980's, but only for the masc. inanim. and neuter genders. Now they're accepted for masc. anim. as well.
Without your question, I wouldn't even have realized that there's something "new" about these forms. They are perfectly equivant to the shorter forms of personal (non-relative) pronouns in the genitive singular, i.e. "jej" and "něj" alongside of "jeho" and "něho" in masc. anim./inanim. as well as neuter. So it seems to make sense that they should also exist when "-ž" is added in the relative pronouns.
Also note that "jenž" and all its forms is a bit formal and not used in casual speech, where we prefer "který" with its simpler declination. In everyday language, we'd say:
- standard: Kde je ten talíř, z(e) kterého jsem jedl?
- common Czech: Kde je ten talíř, z(e) kterýho (j)sem jed?
- or, common Czech: Kde je ten talíř, co (j)sem z něj jed? -- this is used quite frequently
Yeah :) I know, it's a bit of a brain-wrinkler. We actually accept variants of it with standard endings:
"Kde je ten talíř, co jsem z něj jedl?"
I honestly can't say if this counts as standard Czech. It's used in speech a lot, but at the same time, I definitely wouldn't write it anywhere where proper or formal language is expected. I'll give you a few more examples, it's useful to know about this at least passively, because people say it a lot.
- tužka, co jsem s ní psal -- lower register
- tužka, s(e) kterou jsem psal -- neutral register
- tužka, s níž jsem psal -- higher register
Some more examples:
- deník, co do něj psal = deník, do kterého psal (the journal he wrote into)
- film, co jsme na něj koukali = film, na který jsme se dívali (the movie we watched)
- kámoš, co jsem s ním šel na pivo = kamarád, se kterým jsem šel na pivo (the friend I went out to have beer with)
- čaj, cos ho pil = čaj, který jsi pil (the tea you drank)
Less commonly, it's also possible to use "jak" instead of "co":
- Užs viděl ten film, jak jsem ti o něm říkal? = Už jsi viděl ten film, o kterém jsem ti říkal? (Have you already seen the movie that I was telling you about?)
- Líbí se mi ta tužka, jaks mi ji ukazoval. = Líbí se mi ta tužka, kterou jsi mi ukazoval. (I like the pencil you were showing me)
Also note that if the word being pointed to in the relative clause is in the accusative and has no preposition, the pronoun becomes optional (and often left out) in the "co" clause. In this case "co" (or "jak") is simply a lower-register variant of "který":
- čaj, cos pil -- (cos ho pil)
- tužka, cos mi ukazoval -- (cos mi ji ukazoval)
- peníze, co jsem ti půjčil -- (co jsem ti je půjčil) (the money I lent you)
- peníze, co sis půjčil -- (co sis je půjčil) (the money you borrowed)
If the word being pointed to in the relative clause is in the nominative, there is no pronoun to add. Again, then we just have "co/jak" = "který".
- chlap, co sem chodí = chlap, který sem chodí
- kafe, co mi chutná = káva, která mi chutná
- holka, co tu bydlí = holka, která tu bydlí
I just happened upon "...na základě zdravotního postižení, jehož účelem je narušení...." where "jehož" CANNOT be replaced by "jejž". It's the "true" genitive used basically for possession, i.e. "whose".
So, replacing by this shorter form is possible after a preposition -- in which case it's "nějž", not "jejž":
- okno, z nějž se dívám -- the window I'm looking out of
- muž, bez nějž nemohu žít -- the man without whom I cannot live
It's also possible when a verb needs a genitive object, in which case it's "jejž", but in such situations "jehož" is preferred:
- Je to člověk, jejž (jehož) si vážím. -- He's a person I respect.
(From the non-relative "Vážím si ho/jej/jeho.")