This paper focuses on the cognitive foundation of the semantics of English adjectives that denote mental and moral characteristics of human beings. Research into these adjectives seems a challenging task because they denote abstract qualities that cannot be perceived through vision, hearing, or touch; and here a question arises: How are abstract qualities interpreted in English encoded through adjectives? To answer it, this study follows the idea of two-level semantics, i.e. word semantics is treated as a two-level phenomenon that comprises the semantic (external) level and the conceptual (deep) one. This study is the first to address adjectival semantics from this perspective. Here a novel approach to revealing the cognitive foundation of adjectives is introduced: given that adjectives originated from old syncretic items and a word cognitive model forms at the moment of word creation, cognitive models underlying adjectives' semantics are unearthed via analysis of their etymological data. Our contribution is two-fold. First, the approach has revealed that the image schema CONTAINER guides semantics of an array of various adjectives independent of their morphemic structure or date of origin. The examples demonstrate that abstract human qualities are interpreted via the following container features: boundary, container substance, size, hardness/softness of a container shell, etc. The semantics of affixed or compound adjectives appear to stem from the integration of concepts represented by an affix and a root or two roots, respectively. Second, the findings show that the value given to every container feature appears to predetermine the evaluation conveyed by an adjective. Container features tend to possess ambivalent value, realizing the positive or negative one due to the interaction with a frame in which the CONTAINER is incorporated, therefore the same polysemantic adjective may develop both positive and negative meanings. To reveal the whole inventory of cognitive models that govern adjectival semantics in English, further research needs to be conducted.