x1 is a logical 'NOT' statement resulting from the (multiplicative) negation of/applying the 'NOT' operator to x2 (du'u); x1 is the logical negation/negative of x2.
x1 and x2 have mutually opposite truth values. This word is useful for producing "'NOR' statement" from kanxe, for example. See also: na, nai. This is to na or nai approximately as kanxe is to je.