noun, plural hard-ons. Slang: Vulgar.
Origin of hard-on
Also, hard upon, hard by. In close proximity, as in The police were hard on the heels of the thieves, or It was hard upon three o'clock, or Their house is hard by ours. The variants are used less than hard on. [Second half of 1700s]
be hard on. Deal severely with, cause damage to. For example, He asked the teacher not to be too hard on those who forgot the assignment, or That cat has really been hard on the upholstery. [Second half of 1600s] Also see hard time, def. 2.