To get out of what was holding you. Also to mark a character so it is read as itself rather than as an instruction.
Vulgar Latin excappare, ex- (out) plus cappa (a cloak); literally to slip out of one's own cloak and leave a pursuer holding it.
The getting-out of what was holding a thing in place, or the marking of a character so it is read as itself rather than as an instruction. Both senses are a slipping-free from something that would otherwise take hold.