支支吾吾 is a four-character idiom (chengyu) that describes someone speaking hesitantly, evasively, or unclearly — like hemming and hawing in English. It's used when a person avoids giving a direct answer, often because they are nervous, guilty, or hiding something. The literal meaning ('branch branch, I I') doesn't hint at the actual usage, so treat it as a fixed expression meaning 'to be evasive' or 'to mumble'. You'll hear it in contexts like questioning or confrontation.