Word meaning (adjective) expressing or causing grief or unhappiness
The modern meaning is often to be found in Shakespeare, so this makes it especially important to note the many occasions when it does not apply. We need to be on the alert for two senses in particular. One is where the word means "serious, grave, solemn". When the Clown says to Autolycus, "my father and the gentlemen are in sad talk" (The Winter's Tale, IV.iv.308), he does not mean that they are unhappy. The context often provides a clue by providing a synonym or antonym: "Sad and solemn music" says the stage direction at Henry VIII, IV.ii.81; "What was he, sad or merry?" says Cleopatra to Alexas, asking about Antony (Antony and Cleopatra, I.v.50).
The other sense is "dismal, morose, sullen". This is the sense we need when deposed king Richard describes his jailer as a "sad dog" (Richard II, V.v.70), Antony talks about his "sad captains" (Antony and Cleopatra, III.xiii.183), and Puck refers to Hermia as "curst and sad" (A Midsummer Night's Dream, III.ii.439).