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.