It was unfortunate that the train was cancelled, leaving hundreds of passengers stranded.

unfortunate 是由前缀 un-（表示否定）和形容词 fortunate（幸运的）组成的复合词，字面意思是"不幸运的"，引申为"不幸的、令人遗憾的"。 fortunate 本身来自名词 fortune（命运、运气），加上形容词后缀 -ate 构成。