Coupling light and acoustic waves via the nonlinear optical effect of stimulated Brillouin-Mandelstam scattering offers exceptional capabilities for photonic signal processing, narrow-linewidth lasers, sensing, and material analysis. A key parameter for high performance in all these applications is the Brillouin gain. However, current platforms have to overcome the challenges of optical losses, limited interaction length, nonlinear effects, and insertion into practical setups. Here, we demonstrate a Brillouin device based on the reversible freezing of a carbon disulfide-filled liquid-core optical fiber. This approach delivers a giant in-fiber Brillouin gain of 434W-1 m-1. Employing seeded Brillouin spectroscopy, we characterize the spectrum and find a linewidth of 24MHzwhile maintaining low propagation losses in a fully spliced architecture and providing the potential for meter-scale interaction lengths. Leveraging this gain, we realize—as an example application—an optoacoustic memory operating at sub-nanojoule pulse energies—more than two orders of magnitude lower than previous implementations. This power reduction is universal for Brillouin-based fiber applications in general and will enable low-power photonic signal processing, efficient microwave photonics, and high spatial resolution sensing, as well as in-fiber quantum optomechanics-based technologies.