Sad is an adjective that has changed its meaning considerably over time, like other words in the category:changing meanings.
- It began as a Germanic adjective (cognate with Latin satis) meaning 'satisfied', or 'full'. Cf. modern English 'satiated', 'sated' and 'satiety'.
- This developed to mean 'serious'. 'earnest', 'sincere' and 'solemn'; 'settled' and 'steadfast'. (None of these meanings is current, though some are to be found in Shakespeare.) In [[Coleridge's 'Rime of the Ancient Mariner' VII, (in Wordsworth and Coleridge Lyrical Ballads (1798) the wedding guest, having listened to the mariner's improving and moral tale, is described as "A sadder and a wiser man" - a more serious and adult human being.
- The principal use nowadays is 'feeling sorrow', 'grieving' or 'upset'; 'unhappy'. (This sense-development is not, apparently, seen in any other language.)
- This can be transferred: 'sad memories' are things that make the person who remembers them sad, and a 'sad place' is one which contains or arouses such memories. It can also, like 'sorry', be transferred to mean 'deplorable', 'unacceptable' or 'in poor condition': "her clothes were in a sad state"
- In a seemingly related meaning, sad used to be used with colours to mean 'dark', 'dull' or 'dreary'. By itself, it meant 'sombre', 'gloomy' or 'neutral'.
- "Used as a general expression of censure, depreciation, or regret. Originally: exceptionally bad, deplorable, shameful. Later (also): unfortunate, regrettable, sorry, miserable." (OED meaning 6.)
- This has now come to be used in slang to mean 'inadequate', 'unfashionable' - a general tone of derision, with a certain [mock-?] sympathy in it. Writers of academic English should never use the word in that sense.