As other answers point out, this is a term to describe pressure. The most basic technique for creating water pressure involves either storing the water in a raised container (think water towers) or in maintaining a reservoir of sufficient depth to achieve relative height. The higher the height, called "head," the higher the pressure, which is why pressure formulated as "head" is measured in distance (often feet).
The use of the term "head" is appropriate simply because it is a measure of height, and the highest point of anything can be referred to as a head.
Although steam is a different method of generating pressure, the term carries over. As you can see from the chart in Jim's answer, although in the phrase you're asking about the use is colloquial, not technical, it is not just a figure of speech put a defined unit of measurement as well.
A Text Book on Hydraulics - 1906
Section 61 (at the bottom of the linked page):
61. Meaning of "Head" - The word head is used by writers on Hydraulics in a somewhat indefinite way. In all cases it means the
height of a column of water, either actual or ideal. Thus, "head on an
orifice," or on any point of an orifice, has in preceding discussions
been used to designate the vertical height of the free surface of
water above the point under consideration.
Merriam-Webster, entry "Head"
Definition 9a: the uppermost extremity or projecting part of an object