Originally Posted by
Huntsman Spider
It goes all the way back to Gerry Conway's Spectacular Spider-Man run. Long story made short, Tombstone was Robbie's chief tormentor when they were kids, somewhat like Flash Thompson with Peter Parker, only Tombstone got worse as he got older. Even then, Robbie was still so frightened of Tombstone that when he saw him kill someone, he kept his mouth shut rather than turn him in to the police, which allowed Tombstone to make a name for himself with even more killings, becoming a hitman for the New York criminal underworld. Well into adulthood, realizing that Tombstone would never stop unless someone stopped him, Robbie overcame his fear of Tombstone to speak out as a witness to Tombstone's crime(s), but thanks to some corruption in the justice system that Tombstone was able to manipulate, Robbie ended up imprisoned as an accessory after the fact, only to be cleared of that charge and freed later.
The 90s animated series has a somewhat different take on their relationship, where they started off as childhood friends despite young Lonnie Lincoln's penchant for delinquency, only for it to get Lonnie in trouble with the police, whereas young Robbie was able to get away without the police noticing him, partly because Lonnie declined to admit Robbie's part in his crime. As young adults, Lonnie had become a full-fledged criminal and Robbie had become a cub reporter, though resenting that Robbie let him take the fall all those years ago, Lonnie tried to set Robbie up to look like a criminal as well so that Robbie would have no choice but to join him in actual criminality. In their struggle, Lonnie accidentally fell into a vat of chemicals . . . which should have killed him outright, but instead bleached his hair and skin, somehow making said skin as hard as stone and imbuing him with enough physical strength to challenge Spider-Man when he resurfaced as the underworld hitman Tombstone. Making it even more personal was that Tombstone recruited Robbie's disaffected son Randy to be part of his gang, with the plan being to use Randy's membership in his gang as a weapon to ruin Robbie's reputation, family, and life, a plan which Robbie was able to foil with both Spider-Man's help and proving to Randy that he truly did love him and would go to the ends of the Earth for him. Of course, that wouldn't stop Tombstone from setting up Robbie to take the fall for one of his crimes in a later episode, which again forced Peter Parker/Spider-Man to intercede, with help from J. Jonah "Jigsaw" Jameson.
That's about it.