内 (nèi): Inside, internal. Think of the space inside a box (口).
忧 (yōu): Worry, anxiety, concern. This character depicts a person's head (页) with a heart (心) full of sorrow. It's a deep, unsettling feeling.
外 (wài): Outside, external, foreign. The opposite of 内. It's composed of 'evening' (夕) and 'to divine' (卜), perhaps originally meaning divination done outside the home.
患 (huàn): Trouble, disaster, calamity. A 'string' (串) of problems for the 'heart' (心). This isn't a small problem; it's a serious affliction or disaster.
Together, these characters paint a clear picture: 内忧 (nèi yōu) means “internal worries,” and 外患 (wài huàn) means “external disasters.” The idiom combines them to describe a desperate situation where an organization is rotting from the inside while being attacked from the outside.
This is a formal, literary idiom. You won't hear it in casual daily chatter, but it is common in more serious contexts.
Politics and News Media: This is its most common usage. Commentators use it to describe a country facing, for example, domestic economic problems and social unrest (内忧) while dealing with international sanctions or geopolitical tensions (外患).
Business and Corporate World: A CEO might describe their company's situation as 内忧外患 if they are dealing with internal power struggles and low morale (内忧) while facing aggressive competitors and a shrinking market (外患).
Metaphorical Use: While less common, it can be used metaphorically to describe a large family or organization's crisis. For example, the head of a clan might feel they are in a state of 内忧外患 if the younger generation is fighting over inheritance (内忧) while the family business is failing (外患).
Its connotation is always negative and indicates a situation of extreme difficulty and crisis.