Nonlinear wave-mixing in mesoscopic silicon structures is a fundamental nonlinear process with broad impact and applications. Silicon nanowire waveguides, in particular, have large third-order Kerr nonlinearity, enabling salient and abundant four-wave mixing dynamics and functionalities. Besides Kerr effect, in silicon waveguide two-photon absorption generates high free-carrier densities, with corresponding fifth-order nonlinearity in the forms of free-carrier dispersion and free-carrier absorption. However, whether these fifth-order free carrier nonlinear effects can lead to six-wave mixing dynamics still remains an open question until now. Here, we report the first demonstration of free-carrier induced six-wave mixing in silicon nanowires. Unique features including inverse detuning dependence of sixwave mixing efficiency, and its higher sensitivity to pump power, are originally observed and verified by analytical prediction and numerical modeling. Additionally, asymmetric sideband generation is observed for different laser detunings, resulting from the phase-sensitive interactions between free-carrier six-wave mixing and Kerr four-wave mixing dynamics. These discoveries provide a new path for nonlinear multi-wave interactions in nanoscale platforms.