  We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic
dynamics of the infinite-layer cuprate CaCuO2. We find that close to the
(1/2,0) point the single magnon decays into a broad continuum of excitations
accounting for 80% of the total magnetic spectral weight. Polarization resolved
RIXS spectra reveal the overwhelming dominance of spin-flip ($\Delta S = 1$)
characterof this continuum with respect to the $\Delta S= 0$ multimagnon
contributions. Moreover, its incident energy dependence is identical to that of
the magnon, supporting a common physical origin. We propose that the continuum
originates from the decay of the magnon into spinon pairs, and we relate it to
the exceptionally high ring exchange $J_\mathrm{c} \sim J_1$ of CaCuO2. In the
infinite layer cuprates long-range and multi-site hopping integrals are pivotal
and amplify the 2D quantum magnetism effects in spite the 3D antiferromagnetic
N\'eel order.
