Related Clues:

• Like a tough cookie?
• Like dated food
• Unable to react, as helium
• Like day-old donuts
• Hard, as bread
• Like old jokes
• Like 103-Down
• Chemically nonreactive
• Phlegmatic
• Couchbound, perhaps
• Flat, as cola
• Utterly lethargic
• Stuffy, as air
• Chemically quiet
• Lacking originality