adverb, noun
lots of; a ton of; a mountain of
Emphasizes a very large amount, often used in casual speech. Can function as an adverb or noun.
やることが山ほどある。
I have a ton of things to do.
彼は山ほどの本を持っている。
He has piles of books.
たくさん is a neutral, standard word for 'many' or 'a lot'. 山ほど is more emphatic and casual, literally 'as much as a mountain'.
いっぱい is a very common casual word for 'full' or 'a lot'. 山ほど is stronger and more vivid, suggesting an overwhelming quantity.
Compound of 山 (mountain) and the particle ほど (extent, degree), literally 'to the extent of a mountain', used figuratively to mean a huge amount.