Busts of the god Hermes affixed to a quadrangular stone pillar, diminishing towards the base, and between five and six feet in height. They were set up to mark the boundaries of lands, at the junction of roads, at the corners of streets, and so on. The Romans used them also for garden decorations. In later times the block was more or less chiselled into legs and arms.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894