What this suffix does
-brained forms compound adjectives that describe the quality, reliability, or character of someone's mental processes. Most -brained compounds in modern English are critical: scatter-brained, bird-brained, harebrained, feather-brained, crack-brained — each paints a picture of a mind that fails to work clearly, consistently, or reliably. The animal compounds (bird-brained, hare-brained, feather-brained) draw on the popular (often unfair) reputation of small animals for limited intelligence. These compounds are vivid and often humorous, belonging to the satirical and comic registers of English.