Welllll ... Weight is not actually a 'basic' unit at all.
'Mass' is, and its unit is the kilogram.
'Weight' is a 'force' ... a derived unit ... made up
of mass and acceleration. The unit of force is
the (kg-meter/sec²), called the "Newton"
(1 newton) is the force that accelerates (1 kilogram of mass)
at the rate of (1 meter per second) every second.