The language of Qoheleth is characterized by an awkward style and an unusual vocabulary. Past studies have assigned these peculiarities to linguistic factors, assuming that the book reflects an underrepresented dialect or register. The current study aims to expand the boundaries of this discourse by introducing extra-linguistic considerations into the discussion. Qoheleth is the only biblical book that is purely philosophical, focusing on abstract issues such as the purpose of life and the problem of free will. Such philosophical discussions require the use of an abstract terminology. The basic toolkit of any philosopher consists of conceptual phrases such as “time,” “space,” “cosmos,” “humanity,” “meaning,” etc. Yet abstract vocabulary was meager in the Hebrew at the author’s disposal. Paving a pioneering way in the realm of thought, Qoheleth’s author had to create a terminological system capable of expressing his new ideas. This article traces the ways in which the need for a personally-customized philosophical idiom shaped Qoheleth’s language. Exploring the origins of eleven key-terms in the book, this article reveals the dynamics that gave rise to Qoheleth’s personal lexicon. These include generalization and conceptualization of the extant semantic fields of certain terms in order to re-invent them as personal expressions reflecting the author’s philosophy. The author takes advantage of the “linguistic availability” of certain terms, that is, their foreignness or rareness that makes them better-suited, in his view, to bear newly created meanings. Taken together, Qoheleth’s neologisms constitute a personal idiolect, carefully designed to convey the author’s unique thought.