  Heavily doped Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ ($x$=0.19 and 0.26) single
crystals were successfully grown, and investigated by the measurements of
resistivity and anisotropic magnetic susceptibility. In contrast to the
antiferromagnetic insulating ground state of the undoped BaMn$_{2}$As$_{2}$,
the K-doped crystals show metallic conduction with weak ferromagnetism below
$\sim$50 K and Curie-Weiss-like in-plane magnetic susceptibility above $\sim$50
K. Under high pressures up to 6 GPa, the low-temperature metallicity changes
into a state characterized by a Kondo-like resistivity minimum without any
signature of superconductivity above 2.5 K. Electronic structure calculations
for $x$=0.25 using $2\times2\times1$ supercell reproduce the hole-doped
metallic state. The density of states at Fermi energy have significant As 4$p$
components, suggesting that the 4$p$ holes are mainly responsible for the
metallic conduction. Our results suggest that the interplay between itinerant
4$p$ holes and local 3$d$ moments is mostly responsible for the novel metallic
state.
