When combined, the literal meaning is “not (不) enough (足) to be considered (为) strange (奇).” This elegantly forms the idiomatic meaning: something is so logical and expected that it doesn't even register as strange or surprising.
“不足为奇” reflects a cultural appreciation for logic, patterns, and cause-and-effect. In many situations, correctly predicting an outcome is a sign of wisdom and experience. A useful comparison is with the English phrase, “I'm not surprised.” In English, this phrase can often carry a subtext of disappointment, sarcasm, or an “I-told-you-so” attitude (e.g., “He failed the test? Well, I'm not surprised, he never studies.”). “不足为奇,” on the other hand, is typically more neutral and analytical. It's a detached observation about the world's predictable nature. When a Chinese speaker says something is “不足为奇,” they are often subtly highlighting the clear and logical connection between a cause and its effect, rather than passing personal judgment. It aligns with a worldview where understanding the underlying context or principles makes many outcomes seem natural and inevitable, not shocking.
This idiom is widely used in both spoken and written Chinese, from casual conversations to formal news reports.