半路杀出个程咬金

半路杀出个程咬金

Literally means:

Cheng Yaojin comes along on the way

Actually means:

程咬金 was a general in Tang Dynasty, who took two axes as his weapons and often waited in ambush to give people a surprise attack, hence the phrase 半路杀出个程咬金.

It figuratively means an unexpected opponent or trouble appears, changing the situation. Derogatory.

程咬金是唐朝初期的一个大将，用一对板斧为武器，经常先埋伏在半路，然后突然杀出来，所以有“半路杀出个程咬金”的说法。

比喻出现了意外的干扰，使原有的局面发生了变化。具有贬义色彩。