The phrase "shut fast" can be replaced by an array of synonyms. Some substitutes include "close tightly," "clamp shut," "bar securely," "latch firmly," "lock snugly," and "seal tightly." Each of these alternatives suggests a sense of security and stability, leaving no room for escape or intrusion. Such phrases are often used to describe doors, windows, or containers, conveying a strong sense of finality or closure. Overall, these synonyms emphasize the importance of sealing off a space or object, ensuring that nothing can enter or exit until the seal is broken or released.