It is well-known that, generically, the one-dimensional interacting fermions cannot be described in terms of the Fermi liquid. Instead, they present different phenomenology, that of the TomonagaLuttinger liquid: the Landau quasiparticles are ill-defined, and the fermion occupation number is continuous at the Fermi energy. We demonstrate that suitable fine-tuning of the interaction between fermions can stabilize a peculiar state of one-dimensional matter, which is dissimilar to both the Tomonaga-Luttinger and Fermi liquids. We propose to call this state a quasi-Fermi liquid. Technically speaking, such liquid exists only when the fermion interaction is irrelevant (in the renormalization group sense). The quasi-Fermi liquid exhibits the properties of both the Tomonaga-Luttinger liquid and the Fermi liquid. Similar to the Tomonaga-Luttinger liquid, no finite-momentum quasiparticles are supported by the quasi-Fermi liquid; on the other hand, its fermion occupation number demonstrates finite discontinuity at the Fermi energy, which is a hallmark feature of the Fermi liquid. Possible realization of the quasi-Fermi liquid with the help of cold atoms in an optical trap is discussed.Introduction.-An important goal of the modern manybody physics is the search for exotic states of matter. Appropriate examples are spin liquids [1][2][3], Majorana fermion [4][5][6][7][8], topological insulators and semimetals [9][10][11], and others. A peculiar state of one-dimensional (1D) fermionic matter deviating from known types of interacting Fermi systems is the subject of this paper.Let us remind ourselves that the most basic model of the interacting fermions is that of the Fermi liquid. It successfully describes a variety of interacting fermion systems (e. g., electrons in solids, atoms of helium-3) [12]. The approach is based on the Landau's conjecture that both the ground state of a Fermi liquid and its low-lying excitations are adiabatically connected to states of the non-interacting Fermi gas. If the interaction is weak, this hypothesis implies that the perturbation theory in the interaction strength is valid. The latter supplies a theorist with a tool to study specific examples.A known system for which the Landau conjecture fails is a 1D liquid of interacting fermions. The interacting 1D fermions constitute a separate universality class, so-called Tomonaga-Luttinger liquid [13,14]: unlike the Fermi liquid, the Tomonaga-Luttinger ground and excited states have zero overlap with the corresponding non-interacting states, the Tomonaga-Luttinger liquid properties cannot be calculated perturbatively with interaction strength as a small parameter.In 1D the Tomonaga-Luttinger liquid is a generic state of matter. However, recent progress in fabrication and control over the properties of the many-particle systems allows us to look for more fragile types of 1D correlated liquids. Specifically, consider a gas of Fermi atoms in a 1D trap [15]. It is within modern experimental capabilities to vary the effective interaction constant of the opt...