Explanation

Kinematic viscosity ($ u$) is defined as $rac{ ext{momentum}}{ ext{force per unit area}}$, which has dimensions of $rac{( ext{MLT}^{-1})/ ext{M}}{( ext{MLT}^{-2})/ ext{L}^2} = rac{ ext{LT}^{-1}}{ ext{MT}^{-2}/ ext{L}} = ext{L}^2 ext{T}^{-1}$.