1.
huge pile; heaping stack; mountain of things
A large quantity of items piled high, either literally (physical objects stacked up) or figuratively (an overwhelming number of tasks, problems, or issues).
Usage
Most commonly appears in the pattern 山積みになっている (to be piled up) or 山積みにする (to pile up). Also used figuratively for abstract things like 課題 (issues) and 問題 (problems).
Related words
The verb form 山積する is a more formal synonym used primarily in written or business contexts to describe accumulated problems: 問題が山積している.