  Several Cu$^{2+}$ hydroxide minerals have recently been identified as
candidate realizations of the $S=1/2$ kagome Heisenberg model. In this context,
we have studied the distorted system KCu$_3$As$_2$O$_7$(OD)$_3$ using neutron
scattering and bulk measurements. Although the distortion favors magnetic order
over a spin liquid ground state, refinement of the magnetic diffraction pattern
below $T_{N1}=7.05(5)$ K yields a complex helical structure with
$\mathbf{k}=(0.77~0~0.11)$. This structure, as well as the spin excitation
spectrum, are well described by a classical Heisenberg model with ferromagnetic
nearest neighbor couplings. Multiferroicity is observed below $T_{N1}$, with an
unusual crossover between improper and pseudo-proper behavior occurring at
$T_{N2}=5.5$ K. The polarization at $T=2$ K is $P=1.5 \mu$Cm$^{-2}$. The
properties of KCu$_3$As$_2$O$_7$(OD)$_3$ highlight the variety of physics which
arise from the interplay of spin and orbital degrees of freedom in Cu$^{2+}$
kagome systems.
