If someone arrives or does something unannounced, they do it unexpectedly and without anyone having being told about it beforehand.
例句
He had just arrived unannounced from South America...他刚刚从南美来到这里，事先没有打招呼。My first night in Saigon I paid an unannounced visit to my father's cousins.到西贡的第一个晚上，我给父亲的堂兄堂弟们来了个突然袭击。
英英词典释义
Adjective
1. without warning or announcement;
"they arrived unannounced""a totally unheralded telegram that his daughter...died last night"- M.A.D.Howe