  The honeycomb Mott insulator $\alpha$-RuCl$_3$ loses its low-temperature
magnetic order by pressure. We report clear evidence for a dimerized structure
at $P>1$ GPa and observe the breakdown of the relativistic $j_{\rm eff}$
picture in this regime strongly affecting the electronic properties. A
pressure-induced Kitaev quantum spin liquid cannot occur in this broken
symmetry state. We shed light on the new phase by broad-band infrared
spectroscopy of the low-temperature properties of $\alpha$-RuCl$_{3}$ and ab
initio density functional theory calculations, both under hydrostatic pressure.
