I want to make a parody of the famous “for dummies” in Latin.
How could it be translated?
I thought ad nulli to mirror the French version “pour les nuls”, but I’m not sure it actually works.
This is a review/comment on the other suggestions here more than a standalone answer (it started out as a comment to the answer by Kingshorsey).
First of all, in/ad ūsum alicuijus 'for use, enjoyment by X' is clearly the expression to go for here; another option is alicui dēstinātum 'designed, intended for X,' and the two can even be combined.
Next, on to the choice of the nominal, which with in ūsum will appear in the genitive case (all in -ōrum except rudium, tīrōnum), and in the dative before dēstinātum (all in -īs except rudibus, tīrōnibus). And if you're using the latter with another Latin noun, it will need to agree in gender as well, e.g. liber...dēstinātus. If not, leave it neuter:
So do I have any better suggestions? I noticed that Cicero metaphorically used the pairing peregrīnī atque advenae to refer to outsiders to a certain discipline. And the ultimate word for an outsider lacking all familiarity with the customs and culture of the host nation is, of course, barbarus, a word which besides referring to non-Greeks or Latins (i.e. all cultural foreigners) is also synonymous to rudis, incultus and thus standing in opposition to hūmānus, ērudītus. I think therefore that in ūsum barbarōrum would play nicely on this Roman cultural opposition, being both ironic (since that opposition is - ehem - foreign to, and in fact targetting most of us) and unironic at the same time (since we can understand and appreciate it), a tension very much like that in the English 'for dummies'. As such, I think it would make for an excellent translation.
Apart from that, there actually is a specialised condescending word for someone who belongs to the uneducated vulgus, synonymous to indoctus, illitterātus, and that word is idiōta (see Forcellini to get a better sense for its usage and connotations). Its closest English equivalent is 'layman', while 'idiot' is a notorious false friend. The only problem is that it's in no way humorous or colloquial, but rather haughtily educated. It would be phrased as in ūsum idiōtārum and idiōtīs dēstinātum.
A single passage of Cicero gives us three near synonyms worth considering:
imperiti homines rerum omnium rudes ignarique (inexperienced men, unfamiliar with and ignorant of everything)
All three words denote lack of knowledge stemming from lack of experience. All three also have something of the derogatory connotation in "dummies," although raw intellectual capacity isn't at issue.
I'm partial to rudis because it's in the title of one of St. Augustine's treatises: De Catechizandis Rudibus (Catechizing the Uninstructed)
Whichever word you choose, you will need to use it in a particular case that depends on the form of the rest of the title. Perhaps we could riff on a classic series, in usum Delphini (for the use/benefit of the Dauphin).
So, Cooking for Dummies might come out Institutiones Coquinariae, in usum Rudium.
One Latin word worth considering is tiro, meaning a beginner. A beginner is not quite the same as a dummy, but perhaps close enough for many purposes. I'd be happy to use it in a book title along the lines of "Latin for dummies".
To say "for dummies", you need the plural dative of this word: tironibus.
Let me also comment on your suggestion ad nulli. With the preposition ad you would need the accusative, so it would have to be ad nullos. But this means "to nobody". It's quite different to say that the audience is non-expert than to say that it is non-existent!
The simplest expression could be, "stultis" = "for the stupid", from the adjective, "stultus". This reminds me of the amusing, "stulto stultior" = "more stupid than stupid", in which "stulto" is an ablative-of-comparison; "stultior", a comparative.
Much depends on how offensive you wish to be. Joonas's text would be entitled, "tironibus"; mine, "stultis".
Mind, some of the mistakes that I have made would consign me to the latter work.