noun, noun which may take the genitive case particle 'no'
heap; pile; mountain; heaping portion
Describes a large, heaping amount of something, often food served generously. Can be used as a noun or with の to modify another noun.
食堂で山盛りのご飯を頼んだ。
I ordered a heaping portion of rice at the cafeteria.
机の上に書類が山盛りになっている
There is a pile of documents on the desk.
大盛り means a large serving (often a menu size), while 山盛り emphasizes a heaping, mound-like quantity, often beyond a standard large portion.
Compound of 山 (mountain) and 盛り (serving, piling up), literally 'mountain serving'.