The word 'sad' has undergone one of the most dramatic semantic transformations in the English language, shifting from a word meaning 'full' and 'satisfied' to one meaning 'sorrowful' and 'unhappy.' The journey from satiety to sorrow took roughly four centuries and proceeded through a chain of logical emotional associations.
Old English 'sæd' meant 'sated,' 'full,' 'having had enough.' It comes from Proto-Germanic *sadaz (sated, full), from PIE *seh₂- (to satisfy, to have enough). In this earliest sense, 'sad' was essentially positive — it described the pleasant state of having eaten or drunk to satisfaction. The German cognate 'satt' still means 'full' or 'sated' ('Ich bin satt' — 'I've had enough to
The semantic shift proceeded through several stages during the Middle English period. From 'sated/full,' the word moved to 'heavy' (the physical heaviness that follows eating to excess). From 'heavy,' it moved to 'weary' and 'sluggish.' From 'weary,' it moved to 'serious' and 'grave' (a person weighed down by responsibility). And from 'serious/grave,' it finally arrived at 'sorrowful' in the fourteenth century. The connecting thread
The PIE root *seh₂- (to satisfy, to have enough) produced an impressive family of English words through different paths. Through Latin 'satis' (enough), it gave 'satisfy' (to make enough), 'satiate' (to fill to excess), 'saturate' (to fill completely), and, most surprisingly, 'asset' — from Anglo-French 'asez' (enough), from Latin 'ad satis' (to sufficiency); your assets are literally 'your enoughs,' the things you have sufficient of to meet your obligations.
The word 'satire' may also belong to this family, though the connection is debated. Latin 'satura' (a medley, a full dish — as in 'lanx satura,' a plate full of mixed fruits offered to the gods) may derive from the same root, with satire conceived as 'a full plate' of mixed criticisms.