It is in at least 3 ways
1) IF I hold some sort of essentialism or Natural Law etc your relativism has no basis for refuting me. You would be the blind man telling the sighted that "It is impossible that you see what I can't"
2) When my relativism clashes with yours, who prevails? The Stronger? Then it is a matter of will masquerading as something else.
3) It is not logically possible that everything is relative...relative to WHAT?
And in the field of morals if all we do is make up our morals (rather than discovering them) why care at all about them?
- Even the definition shows an obvious lack of thinking : the doctrine that knowledge, truth, and morality exist in relation to culture, society, or historical context, and are not absolute.
If this is so, then how does anyone get outside their own culture? Something can very well owe its form to context but still be invariant: Thou shalt not kill