noun. 1A cap for the head. 2A marking resembling this on the head of an animal. 3(usually as one word) that part of a book's binding which covers the headband.

What is a head cap?

: the covering leather at the head and foot of the backbone of a hand-covered book shaped over the headbands.

Is cap an actual word?

History lesson: In the 1940s, according to Green's Dictionary of Slang, to cap is evidenced as slang meaning “to surpass,” connected to the ritualized insults of capping (1960s). These terms appear to be rooted in the sense of cap as “top” or “upper limit.”