Acoustic oscillations of metal nanoparticles can be used to study the properties of liquids at GHz frequencies and nanometer length scales. We use time-resolved spectroscopy to probe the dynamics of the metal nanoparticle oscillations utilizing a pump-probe technique. The incident pump laser pulse heats the nanoparticles leading to expansion and impulsive excitation of vibrations of the nanoparticles. The oscillations produce shifts in the plasmon resonance, which are monitored by measuring the change in absorption of a second weak broadband probe pulse. In our experiment, we immersed a sample of highly monodisperse gold bipyramids in water-glycerol mixtures from which we determined the damping resulting from the structure-liquid interactions. Performing these measurements over a range of temperatures provides a means to vary the fluid properties of a given water-glycerol mixture. Viscous damping could account for the measured results at low glycerol concentrations and sufficiently high temperatures but failed to describe the damping for high glycerol concentrations and sufficiently low temperatures. Accounting for the viscoelastic nature of the liquid mixtures mostly resolved the discrepancies, but consistently overestimated the degree of damping. Ultimately, allowing for a finite slip length produced good agreement with the measured damping rates. Our results show that standard assumptions in the fluid mechanics of simple liquids – a purely viscous response and the no-slip boundary condition – must be revisited at short length scales and fast time scales.