sad

The word 'sad' originates from the Old English 'sæd', which meant 'sated, full, weary'. This sense of being weary or heavy evolved from the Proto-Germanic 'sadaz' ('sated'), which in turn derives from the Proto-Indo-European root 'seh₂-' ('to satiate'). The semantic journey from a physical state of being full or weary to the emotional state of sorrow is a classic example of metaphorical extension. Over time, the primary meaning shifted from physical satiety to a heavy, weary heart, eventually solidifying into its modern abstract sense of unhappiness or sorrow by the 14th century.

The word 'sad' is a simplex word of Germanic origin. It functions as a whole, unanalyzable unit in modern English.