adverb
in swarms; in great crowds; in huge numbers
Describes people, animals, or things appearing or gathering in large, often lively numbers. Colloquial and somewhat emphatic.
祭りには人がわんさか集まっていた。
People had gathered in swarms at the festival.
adverb
Used for an abundant amount of something, often with a sense of overflowing or more than enough. Can be used for both countable and uncountable things.
畑にはトマトがわんさか実っている。
Tomatoes are growing plentifully in the field.
The origin is uncertain. It is a colloquial adverb that may be sound-symbolic, evoking a bustling, swarming image.