  We conducted a multi-wavelength, multi-instrument observational
characterisation of the candidate free-floating planet
CFBDSIR~J214947.2-040308.9, a late T-dwarf with possible low-gravity features,
in order to constrain its physical properties. We analyzed 9 hours of X-Shooter
spectroscopy with signal detectable from 0.8--2.3$\mu$m, as well as additional
photometry in the mid-infrared using the Spitzer Space Telescope. Combined with
a VLT/HAWK-I astrometric parallax, this enabled a full characterisation of the
absolute flux from the visible to 5$\mu$m, encompassing more than 90\% of the
expected energy emitted by such a cool late T-type object. Our analysis of the
spectrum also provided the radial velocity and therefore the determination of
its full 3-D kinematics. While our new spectrum confirms the low gravity and/or
high metallicity of CFBDSIR2149, the parallax and kinematics safely rule out
membership to any known young moving group, including AB~Doradus. We use the
equivalent width of the KI doublet at 1.25$\mu$m as a promising tool to
discriminate the effects of low-gravity from the effects of high-metallicity on
the emission spectra of cool atmospheres. In the case of CFBDSIR2149, the
observed KI doublet clearly favours the low-gravity solution. CFBDSIR2149 is
therefore a peculiar late-T dwarf that is probably a young, planetary-mass
object (2--13Mjup, $<$500Myr) possibly similar to the exoplanet 51Erib, or
perhaps a 2--40Mjup brown dwarf with super-solar metallicity.
