The thermal diffusivity in the ab plane of underdoped YBCO crystals is measured by means of a local optical technique in the temperature range of 25-300 K. The phase delay between a point heat source and a set of detection points around it allows for high-resolution measurement of the thermal diffusivity and its in-plane anisotropy. Although the magnitude of the diffusivity may suggest that it originates from phonons, its anisotropy is comparable with reported values of the electrical resistivity anisotropy. Furthermore, the anisotropy drops sharply below the charge order transition, again similar to the electrical resistivity anisotropy. Both of these observations suggest that the thermal diffusivity has pronounced electronic as well as phononic character. At the same time, the small electrical and thermal conductivities at high temperatures imply that neither well-defined electron nor phonon quasiparticles are present in this material. We interpret our results through a strongly interacting incoherent electron-phonon "soup" picture characterized by a diffusion constant D ∼ v 2 B τ , where v B is the soup velocity, and scattering of both electrons and phonons saturates a quantum thermal relaxation time τ ∼ /k B T.thermal diffusivity | bad metals | electron-phonon T he standard paradigm for transport in metals relies on the existence of quasiparticles. Electronic quasiparticles conduct electricity and heat. Phonon quasiparticles, the collective excitations of the elastic solid (here, we discuss acoustic phonons) also conduct heat. Transport coefficients, such as electrical and thermal conductivities, can then be calculated using, for example, Boltzmann equations (1). However, such an approach fails when the quasiparticle mean free paths become comparable with the quasiparticle wavelength. For electrons, it is the Fermi wavelength (2-4), whereas for phonons, it is the larger of the interatomic distance or minimum excited phonon wavelength (5, 6). Understanding transport in nonquasiparticle regimes requires a new framework and has become a subject of intense theoretical effort in recent years, triggering an urgent need for experimental results that can shed light on such regimes. In particular, in ref. 7, the diffusivity was singled out as a key observable for incoherent nonquasiparticle transport, possibly subject to fundamental quantum mechanical bounds.In this work, we report high-resolution measurements of the thermal diffusivity of single-crystal underdoped YBCO6.60 (an ortho-II YBa2Cu3O6.60) and YBCO6.75 (an ortho-III YBa2Cu3O6.75). We are particularly interested in the anisotropy of the thermal diffusivity as measured along the principal axes a and b (which is the chain direction) in the temperature range 25-300 K. We use a noncontact optical microscope (see Fig. S1) to perform local thermal transport measurements on the scale of ∼10 µm, hence avoiding inhomogeneities, particularly twinning and grain boundary effects. Our principal experimental results are as follows. (i) The measured thermal diffusivity is...