никой

ни (ni) +‎ кой (koj)

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ни́кой • (níkoj)

1. nobody (as a pronoun)
2. not one (as a pronominal adjective)

The accusative, dative and prepositional forms are used only when ни́кой is on its own (not in front of a noun) and refers to a person. The dative form is dated and is replaced by на (na) + the prepositional form (На ни́кого да благодаря). In informal Bulgarian the nominative form is often used instead of the accusative and prepositional ones (Ни́кой питаш, с ни́кой разговарям). This usage is not considered standard.

• “никой”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Sofia: Bulgarian Academy of Sciences, 2014
• “никой”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Chitanka, 2010

ни (ni) +‎ кой (koj)

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никой • (nikoj)

1. (archaic, literary) not any, no

   Synonym: никако́й (nikakój)

   Antonym: кой (koj)

   нико́им о́бразом ― nikóim óbrazom ― in no way

• Now only found in a few set phrases and expressions.
• With prepositions, нико́й (nikój) is split into ни (ni) and кой (koj), with the preposition in between, such as in ни в ко́ем слу́чае (ni v kójem slúčaje, “in no case / not in any case”).

• Vasmer, Max (1964–1973) “никой”, in Oleg Trubachyov, transl., Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress\