  Magnetoelectric (ME) effect is recognized for its utility for low-power
electronic devices.Largest ME coefficients are often associated with phase
transitions in which ferroelectricity is induced by magnetic order.
Unfortunately, in these systems, large ME response is revealed only upon
elaborate poling procedures. These procedures may become unnecessary in
single-polar-domain crystals of polar magnets. Here we report giant ME effects
in a polar magnet Fe2Mo3O8 at temperatures as high as 60 K.Polarization jumps
of 0.3 {\mu}C/cm2, and repeated mutual control of ferroelectric and magnetic
moments with differential ME coefficients on the order of 10$^4$ ps/m are
achieved.Importantly, no electric or magnetic poling is needed, as necessary
for applications.The sign of the ME coefficients can be switched by changing
the applied "bias" magnetic field. The observed effects are associated with a
hidden ferrimagnetic order unveiled by application of a magnetic field.
