The term 'Sing with Closed Lips' refers to a type of singing technique in which the singer does not open their mouth, but instead hums or hummed the melody. The crossword solutions for this term are HUM (3 letters) and HUMMED (6 letters). HUM is a verb that means to make a low, continuous sound with the lips closed, while HUMMED is the past tense of the same verb. Singing with closed lips is a popular technique used by many singers, as it allows them to create a unique sound without having to open their mouths.