The paper considers the historical and onomastic aspects of the appearance of (1) the ethnonyms 'Ceh' and 'Bohem', (2) the appellative 'ceh', and (3) the possessive adjectives 'ceski, -a, -o' and 'bohemski, -ska, -sko' in diachrony and synchrony in South Slavic languages (particularly Serbian and Croatian). The old legends and foreign historical books claim that the origin of the old Serbs and Croats was the state of Bojka (Bohemia), the state named after the Celtic tribe Boji who lived on the territory of the present-day Czech Republic. The latter historical and cultural connections among these Slavic peoples conditioned the appearance of the ethnonyms 'Bohem' and 'Ceh' and the possessive adjectives 'ceski' and 'bohemski' in various phonetic and morphological forms in the old writings, books and onymy. The linguistic literature makes a distinction between the ethnonym 'Ceh' and the appellative 'ceh' (a small, fat man). The contemporary Serbian and Croatian language show the following situation: (1) only the ethnonym 'Ceh' is used, (2) the use of the appellative 'ceh' had completely faded away (although other foreign ethnonyms have the polysemy), and (3) from the ethnonym 'Bohem', through French, the appellative 'boem-bohem' has been produced, denoting 'an artist who lives in a careless and unconventional way' and also the linguistic terms 'bohemistika' (the science about the Czech language) and 'bohemista' (a Czech language expert).