Phrase "hard-on" (not penis)

[Alfred001]

I've heard this phrase used often to describe a person, most recently in the Pope of Greenwich Village where a guy was described as an "Irish hard-on."

What does this mean? If its just an insult without particular meaning, which I suspect it might be, then what is the etymology of it?

[tailor STATELY]

http://dictionary.reference.com/browse/hard--on :
2. A severe and intolerant person, esp a martinet leader or superior : Stone is turning into a world-class hard-on (1980s+)

Ta ! (short for tarradiddle),
tailor STATELY

[Lokasenna]

Perhaps a mishearing of 'hard man'?

[bounty]

or a morphing from "hard 'un" maybe?

[Alfred001]

http://dictionary.reference.com/browse/hard--on :

Ta ! (short for tarradiddle),
tailor STATELY

Thanks!