The term 'เสียด้วย (sǐa dûay)' is used in Thai to express feelings of regret, disappointment, or sorrow, similar to the English phrases 'unfortunately' or 'sadly'. It is often used at the beginning or end of a sentence to add a tone of lament or regret to a statement. It can be used with a wide range of verbs, adjectives, and phrases to express various types of unfortunate situations or negative emotions.