After living in Andalucia, Spain, for a number of years I realised my use of "hais" for the second person informal plural (vosotros) of haber instead of "habeis" is incorrect. I don't know where I picked it up. Googling shows me that it is used in some isolated areas of Spain. Is it used in Latin America? Is it used only by country bumpkins or is it recognized as acceptable slang?
I am not a linguist, but I am a user of the word "hais". I think its origin is related to the Leonese "heis". My family comes from Extremadura, a land with historical Leonese influence. But I was born in the Basque Country and I never realised the fact that "hais" is not the Spanish standard word until a teacher of Latin read my translation of "habḗtis".
It is not standard Spanish and I've likewise never heard it.
That said, many regions of Spain spoke (and/or speak) languages other than Castilian and, as a result, their Castilian can be sometimes strongly influenced by those. Note their conjugations of haber/haver:
Language | Conjugation | Note ------------------+-------------------+--------------- --- -- - - - Asturian | vós habéis | Aragonese | busatros ez | Catalan/Valencian | vosaltres heu | also, less commonly, “haveu” Galician | vós havedes | Leonese | vosoutros heis | possibly “hais” in modern times Mirandese | vós heis | Portuguese | vós havéis | also, less commonly, “heis”
So, it is certainly possible that you've heard it if the person is from León, Aragón, a Catalonian/Valencian-speaking region, or along the Portuguese border, who is not used to haber being regular for vosotros. The E/A vowels can alternate a good bit from Castilian, especially with Leonese and Aragonese (and my Leonese source is from a century ago) so it's not inconceivable that a vowel swap could have changed heis -> hais, or it developed as regularized irregular form based on has (or han, etc.) -> hais.
I doubt seriously the average Spanish speaker would hear hais as being haber for vosotros. Most likely, they'd probably hear it as a mispronunciation of has. If they did recognize it for whatever reason, though, their view of it will probably vary greatly by how they were familiarized with it, whether they speak one of the languages in question, etc.
In Latin America, I can't imagine it being used or recognized at all in tuteante (except as a mispronunciation of has). I believe the voseante regions generally have has ~ habés ~ habé(i), so I doubt hais would be used or really understood by any of them, either.
So basically, don't use it, because even if people recognize it (unlikely), you won't be able to necessarily predict how they'll interpret it sociolinguistically.
Our Chilean colloquial voseo verbs (estái, tenís, decís) come from the archaic voseo forms, the same as those of "vosotros" (estáis, tenéis, decís, sois, ibais, vierais):
Archaic voseo → Current Chilean voseo
vos cantáis → vos cantái / tú cantái
vos queréis → vos querís / tú querís
vos tenéis → vos tenís / tú tenís
vos decís → vos decís / tú decís
vos ibais → vos ibai / tú ibai
vos vais → vos vai / tú vai
vos sois → vos soi / tú soi
(Using "vos" in Chile is considered excessively informal and, in some cases, offensive and even vulgar. It's always pronounced "voh".)
Following this logic, "vos habéis" would become "vos habís" / "tú habís", but this form is extremely rare. Instead, we use "vos hai" / "tú hai":
¿Cómo hai estado? (How have you been?)
¿Hai cachado que...? (Have you noticed that...?)
Tú nunca hai hecho eso. (You have never done that)
¿Qué te hai creído? (Who do you think you are?)
It's not incorrect, it's just colloquial, like any other Chilean voseo verb (in fact, it would sound weird and snob if the tuteo form "has" were used in a colloquial context instead of "hai").
And since all "-áis" ending verbs became "-ái", our current "hai" should come from a hypothetical "hais":
vos *hais → vos hai / tú hai
But I'm not sure if the transition was habéis→habís→hai or habéis→hais→hai. This is the only case where I can think of something related to an "hais" here in Latin America.
I have lived in Sevilla (capital of Andalusia) for almost 28 years and I think I have never heard * hais, which is obviously incorrect and anyone with some basic education will stigmatize as vulgar.
Some poorly educated people do say * habemos instead of hemos. But I don't think people say * hais.