One of the tenets of Ranterism in seventeenth century England. It came from the idea of predestination, which stated that God had chosen his elect who would be saved, regardless of their behaviour. Some people, such as the Ranters, interpreted this to mean that any behaviour was morally acceptable, as you were a member of the elect however you behaved. This was not a freedom to sin, but the idea that "through faith and God's freely-given grace, the elect were freed from ordinary moral laws" ('People, Power and Politics: Was there a mid-seventeenth century English Revolution?' by Angela Anderson).
Logically, this idea is the natural extension of predestination. You can see the logic that leads to it: if one is saved by God's grace and God's grace alone, behaviour is irrelevant. However, it was this idea, more than any other, that meant that the Ranters were seen as a lunatic fringe and heretics. Quite possibly it was their antinomianism that prompted the Blasphemy Act 1651, which is the Act credited with destroying the Ranter 'movement' (if one existed in the first place).
Antinomianism varied among those classified as Ranters. Some seemed to accept it but not do anything with this 'freedom' (Joseph Salmon, if memory serves, was called a Ranter but more interested in pantheism), whereas others are notorious for their behaviour, such as Lawrence Claxton. It should also be noted that the worst excesses under antinomianism are documented in anti-Ranter pamphlets and in the 'confessions; of 'ex-Ranters', such as Claxton's 'The Lost Sheep Found.'