This is an as near as I can tell exact British equivalent of U.S. “tough guy,” usually used in a sporting or criminal context. The quintessential hard man is footballer-turned-actor Vinnie Jones. The OED gives no definition or etymology, but the phrase appeared as early as 1984, in this capsule movie review in The Guradian.
“Sequel again features Gene Hackman as maverick, hard-man cop Popeye Doyle, back on the trail of his old drugs-czar adversary Fernando Rey.” (The Guardian, 1984)
Some other examples (taken from OED citations in other words’ definitions):
“There was no room to express love and only space for one kind of man: the hard man, the man’s man.” (Face, 1995)
“To his prison mates Archie was a swaggering hard man who never let a sliver of emotion through the tough exterior he had built against the world.” (Evening News [Edinburgh], 1998)
“Self-styled Hampstead hard man..is actually just a big-mouthed wet.” (Q, February 2003)
Historically, to the extent the the phrase could be found in the U.S., it was in phrases like “You’re a hard man to track down” or in the off-color Mae West chiasmus “A hard man is good to find.” I had assumed I would never encounter the tough-guy meaning here. But I assumed wrong. In a column about (American) footballer Jim Brown in yesterday’s New York Times, sports columnist George Vecsey wrote, “His aging high school teammates still shudder from the dreaded Tuesday tackling drills and know him as a hard man in public life.”
Figures it would be Vecsey, a soccer fan and a man of the world. I don’t expect to come across it again.
notoneoffbritishisms.com 
The quintessential UK hard man was in fact Johnny Dexter: http://en.wikipedia.org/wiki/Johnny_Dexter
I second that.
Norman Tebbitt (a.k.a. ‘the Chingford skinhead’ or ‘a semi house trained polecat’) was the hard man of British politics.
OMG, I definitely second this one. How could anyone forget Tebbitt…
“hard” (in the sense of “violent” or “tough”) goes back to at least the ’70s in the UK, usually in the context of the taunt “come and have a go, if you think you’re hard enough” against someone threatening a fight.
Looks like you’ve antedated the OED by eight or so years, there, Sean.
I’m fairly sure “hard man” goes back earlier than the 80s; there’s a reference to a Scottish play from the 70s and a common Irish phrase of greeting for a friend or acquaintance is “Ah, the hard!” (an abbreviation of “Ah, the hard man!” meaning “Look who’s here”, more or less).
It may be related to the Irhsh (and possibly Scots) phrase “a or the hard chaw” (meaning a tough case, a though person, a thuggish type) – “chaw” being dilacetal for “chew”
This is a variation on a “hard nut”, which goes back to the 19th century. A hard man, in Brit English at least, might also be described as “a bit of a nutter”, without necessarily linking his behaviour to mental illness.
Hard Man definitly goes back earlier than 80s, there is a Scots play called “The Hard man” about, well, a Hard Man.
I think there are two meanings of ‘hard man’. E.g. ‘you’re a hard man, Smith’ means that Smith is mean or ruthless, but not necessarily physically violent – for example, he might have refused help or just sacked someone. But ‘Smith’s a real hard-man’ would mean to me that he was a tough guy in the physical sense. I think the hyphen is important.
come and have a go if you think you’re hard enough.
Human Leauge – Love Action from Dare
You might as well resign yourself
To what you’re going through
If you’re a hard man or if you’re a child
It still might get to you
Lyrics from eLyrics.net
1611 A.D, “For I knew that thou art a hard man…” Gospel According to Matthew, chapter 25. Meaning: austere, rigid, stern, serious.
According to the dictionary on my Mac there is the single word…
Hardman
noun (pl. hardmen) informal
a tough, aggressive, or ruthless man.
…which is given as both American and British English.
There is a subtle variation of the pronunciation of hard man (combined with context) which is more akin to Edward son of Arthur’s Biblical quote meaning unflinchingly austere or stern as opposed to the physically robust hard man.