'There is no mother but loves her child, 'Here 'But' is-

In the context of the sentence "There is no mother but loves her child," the word "but" is not functioning as a negative relative pronoun.

A relative pronoun is a type of pronoun that introduces a relative clause, which modifies a noun or pronoun in the main clause of a sentence. Examples of relative pronouns include "who," "whom," "that," and “which.”

On the other hand, in this sentence, "but" is functioning as a coordinating conjunction, connecting two independent clauses. It is not introducing a relative clause.

Furthermore, "but" is not acting as a negative word in this sentence. Instead, it is being used in the sense of "except" to emphasize that all mothers love their children, despite the negative statement that “there is no mother.”