  We present the stellar radial velocity analysis of the central 1x1' of the
young massive Small Magellanic Cloud star cluster NGC 346. Using VLT/MUSE
integral field spectroscopy in combination with Hubble Space Telescope
photometry we extract 103 spectra of cluster member stars suited to measure
accurate line-of-sight kinematics. The cluster member stars show two distinct
velocity groups at v1 = -3.3 (+0.3/-0.2) km/s and v2 = 2.6 (+0.1/-0.1) km/s,
relative to the systemic velocity of 165.5+/-0.2 km/s, and hint for a third
group at v3 = 9.4 (+0.1/-0.1 km/s. We show that there is neither a correlation
between the velocity groups and the spatial location of the stars, nor their
locus on optical color-magnitude diagrams, which makes the stellar velocity a
key parameter to separate individual stellar components in such a young star
cluster. Velocity group 2 shows clear rotation with Omega2 = -0.4 +/- 0.1
1/Myr, corresponding to -4.9+/-0.7 km/s at radial distance of 10 pc from the
center, a possible remnant of the formation process of NGC 346 through the
hierarchical collapse of the giant molecular cloud. The ionizing gas has lost
any natal kinematic imprint and shows clear expansion, driven by far ultra
violet fluxes and stellar winds of the numerous OB stars in the cluster center.
The size of this expanding bubble and its expansion velocity of 7.9 km/s is in
excellent agreement with the estimate that the latest star formation episode
occurred about two million years ago.
