It was a thinly-veiled threat, so far as Ariadne could see. There was no way it couldn't be.

So she bit her tongue at that point, deciding that was as far as she was going. Cobb had never been able to threaten her with death; she had willingly walked into Limbo, knowing full well what might happen. But this? This was a legitimate method of killing her off in a potentially excruciating way.