To the best of our knowledge, the issue of idiomaticity is considered to be one of the greatest challenges in lexicology. It is due to the fact that English is an analytical language. Despite a wide range of idiomatic categories, the notion ‗idiom‘ has been mostly used without pattern distinction. Consequently, the present article aims at examining the unpredictability of the lexical unit development and the idiomatic expression diffuse rule boundaries that lead to the diversified phenomenon of idiomaticity. Thus, to face the vast number and great diversity of word-combinations, we are going to analyse them according to the following semantic criteria: 1) connotative versus non-connotative word-combinations; 2)cliché-ed expressions; 3) phraseological units proper; 4) word-combinations with extra linguistic peculiarities; and 5) culture-bound word-combinations. Another idiom categorisation is thematically determined. It represents not only the subject matter but at the same time the cultural and sociolinguistic features, which are distinctive to a certain community. We intend to lay emphasis on the thematic clichés which have the British, American and Romanian components. In conclusion, it is important to mention the emergence of a sociolinguistic phenomenon, which come up with names and referents, lexicon and syntax. In consequence, the outlined semantic criteria of lexical-phraseological categorization can help the learner to concentrate on facets of complex lexical units and pin down the errors in phraseological usage.