In Greco-Roman archeology, a Herm is a stone, bronze, or terra-cotta marker--originally placed at a crossroads or at territorial boundaries. In classical Athens, homeowners would erect Herma outside entrances of their houses for good luck.
These stone carvings consisted of a bearded human head (ie, of the god Hermes or Mercury) set on top of a rectangular or square stone column (typically between 1 & 2 meters high) with no arms or legs but a prominent phallus carved to protrude about halfway up the column.
By classical times, before taking long journeys, wayfarers would anoint and rub the Herm's phallus with olive oil as a libation to HERMES/MERCURY, the God of Travel.
In historical literature (such as Alcibiades) it was suggested that vandalism of a Herm was considered one of the most impious acts imaginable among classical pagans.