noun or participle which takes 'suru', intransitive verb, adverb
be surprised; be startled; be amazed
Mimetic word expressing sudden surprise or fright. Can be used as a noun, a suru-verb, or adverbially.
大きな音にびっくりした。
I was startled by the loud noise.
彼女の変わりようにびっくりしている。
I'm amazed at how much she has changed.
noun-modifying (irregular)
surprise (as in surprise party)
Prenominal use, modifying a noun to indicate something unexpected or surprising.
友達がびっくりパーティーを開いてくれた。
My friends threw me a surprise party.