Where does “Гвинеjа” come from?

Гвинеjа (Serbo-Croatian) comes from Portuguese Guiné, from English -ia, from Latin -ia, from Latin -ius, from Proto-Italic -jōs, from Proto-Indo-European -yós — Creates adjectives from noun or verb stems.

Ancestry of “Гвинеjа”, step by step

Гвинеjа traces back through two separate lines of ancestry.

via Portuguese Guiné

StepLanguageWordMeaning
1PortugueseGuinéGuinea
2English-iaUsed in forming names of countries, diseases,...
3Latin-iaUsed to form a feminine abstract noun, usually...
4Latin-iusforming adjectives from nouns; nominative neuter...
5Proto-Italic-jōsForms comparative adjectives
6Proto-Indo-European-yósCreates adjectives from noun or verb stems

via Italian Guinea

StepLanguageWordMeaning
1ItalianGuinea
Every word from Proto-Indo-European -yósEvery word from Latin -iaEvery word from Latin -ius