The word "大概" is formed through a logical and conceptual combination of its two component characters. The first character, "大" (dà), means "big" or "great", and the second, "概" (gài), carries the core meaning of "outline", "generality", or "the main points". When combined, the term literally suggests a "big outline" or the "major brushstrokes" of something. This figurative image, of seeing only the large, essential features rather than the fine details, naturally gives rise to its modern meanings of "approximate", "general idea", or "probably", as it conceptually represents a broad, non-specific view of a subject.