The cognitive approach that studies the mental processes involved in the perception and conceptualization of the world, usually focuses on metaphor and metonymy. In this perspective, they are interpreted as different cognitive mechanisms, formed during the interaction of two conceptual domains (SOURCE and TARGET) based on associations of similarity (metaphor) and contiguity (metonymy). There is, moreover, the opposite view that these phenomena are part of the same cognitive continuum and create hybrid forms with a different « specific weight » of similarity and contiguity. This point of view contributes to the study of a new phenomenon called “metaphtonymy” which represents an amalgam of metaphorical and metonymic relations. The genesis of metaphtonimical relations is explained by three facts, namely: all metaphorical conceptualization is preceded by the metonymic one, which is linked with human consciousness to perceive entirely the reality; contiguity may take partly the form of similarity and vice versa; as conceptual phenomena, metaphor and metonymy can belong to the same conceptual domains and, therefore, interact in the conceptual framework. The analysis of the corpus shows that the phenomenon of metaphthonymy is quite widespread during the creation of neophraseological units in modern French of which we have found two types: some represent the metonymic expansion of the SOURCE and the others – that of the TARGET.