The article deals with the phenomenon of semantic diffuseness on the material of two word associations with characterising semantics: the lexical class of nouns that name a person by virtue of their knowledge, skills and experience, and the lexico-semantic group of adjectives with the general meaning ‘unique/exceptional.’ The study was conducted in several stages, including continuous sampling and classification of the selected units, identification of nuclear units and the analysis of their semantics, as well as the peculiarities of use in the Russian National Corpus and other Internet sources. Verification of the results obtained was carried out by analysing Internet users’ reflexive statements, surveying native speakers, and generalising relevant characteristics of the studied lexical units. Two types of semantic diffuseness have been identified: ambiguity of monosemantic words and ambiguity of polysemantic words. The lexical class of nouns that name a person by virtue of their knowledge, skills, experience features both ambiguity types, while adjectives with the meaning of uniqueness/exclusiveness are characterised by only the second type of ambiguity. Adjectives are noted to demonstrate cognitive-pragmatic syncretism, when one denotate is associated with cognitive and pragmatic signifiers. In the linguistic consciousness of Russian native speakers there is a stereotypical representation in relation to both types of word combinations. The stabilityof stereotypical representations is supported and strengthened by the typified combinability of the considered units, their prepositional-and-case models and the presence of idiomaticity.