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1. slang An erection.
2. rude slang A strong desire or affinity for something. Chrissy had a real hard-on for that band, but now she thinks they're sell-outs.
hard on someone
harming someone's feelings; demanding much from someone. I wish you wouldn't be so hard on me. So I make mistakes. I never said I was perfect.
1. 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]
2. 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.