As a Noun:
1. Something that happens unexpectedly or without warning.
As a Verb or Adjective:
1. To happen suddenly or as a surprise. English = (be) unexpected.
2. To appear in a place unexpectedly or to attend an event unexpectedly or without having been invited. Idiomatic English = show up, pop up.
1. Used next to a verb to mean the action happens suddenly and as a surprise. English = unexpectedly.