taru-adjective, adverb taking the 'to' particle
in heaps; piled up; in piles
Describes things lying or being piled up in heaps, often with a sense of disorder or neglect. Used with と and often modifies verbs like 積む, 置く, or 死ぬ (corpses). Literary and somewhat formal.
戦場には死体が累々と横たわっていた。
Corpses lay in heaps on the battlefield.
机の上に書類が累々と積まれている。
Documents are piled up in heaps on the desk.
山積 means 'mountainous pile' and is more common in modern Japanese, often used for work or problems. 累々 is more literary and emphasizes scattered heaps.
Reduplication of 累 (るい), meaning 'to accumulate' or 'pile up'. The exact historical derivation is uncertain, but the form is used in classical and literary Japanese.