Literary and historical register. Common in classical texts and historical dramas where loyal officials are undone by 谗言 whispered to the throne. In everyday modern speech, prefer 坏话 (huàihuà) or 闲言碎语 (xiányán-suìyǔ).
memory
The character 谗 contains 言 (speech) on the left and 毚 (a hare poking its head out, suggesting sneakiness) on the right — sneaky speech is exactly what 谗言 is.
