An act of treachery, perpetrated on a previous partner in a deceit.
The term 'double-cross' has been used in various contexts for many centuries, usually as a straightforward reference to the shape of two crosses, as in the architectural design of cathedrals for example. That meaning is unrelated to the current figurative 'cheating' usage of 'double cross', which dates only from the late 18th century.
To find the origin of the expression 'double cross' as it is now used, we need to look first at one of the many meanings of the noun 'cross'. From the mid 1700s, a 'cross' was a transaction that wasn't 'square', i.e. not honest and fair. The term was most often used in a sporting context, where a cross was a match that was lost as a result of a corrupt collusory arrangement between the principals involved. You might expect that a 'double cross' was a deceit in which two parties collude in a swindle and one of them later goes back on the arrangement, crossing both the original punters and his erstwhile partner in crime. Although that is the case, the term 'double' doesn't here mean simply 'two times'. 'To double' had long been used to mean 'to make evasive turns or shifts; to act deceitfully'. This derives from the imagery of someone doubling back over a previous route. This 'doubling' gave rise to the term 'double dealing', which has been used since the early 1500s to refer to someone duplicitously saying one thing and doing another, for example, a 'double agent'.