  We propose a general theory of anisotropic band flattening in moir\'e systems
at the $\Gamma$ valley. For a two-dimensional semiconductor with a rectangular
unit cell of $C_{2z}$ or mirror symmetries, we find that a larger effective
mass anisotropy $\eta=m_y/m_x$ of the valence or conduction bands in the
monolayer will have a stronger tendency to be further enhanced in its twisted
bilayer. This gives rise to strong anisotropic band flattening and correlated
physics in one dimension effectively. We predict twisted bilayer black
phosphorus (tBBP) has giant anisotropic flattened moir\'e bands
($\eta\sim10^4$) from ab initio calculations and continuum model, where the low
energy physics is described by the weakly coupled array of one-dimensional
wires. We further calculate the phase diagram based on the sliding Luttinger
liquid by including the screened Coulomb interactions in tBBP, and find a large
parameter space may host the non-Fermi liquid phase. We thus establish tBBP as
a promising and experimentally accessible platform for exploring correlated
physics in low dimensions.
