1. Showing sustained enthusiastic action with unflagging vitality
   "unflagging pursuit of excellence"
   - indefatigable, tireless, unwearying 
2. Unceasing
   "unflagging courtesy"
   - unfailing

See also: constant, energetic

• low comedy
• Low Countries
• low explosive
• low frequency
• low gallberry holly
• low gear
• Low German
• Low Latin
• low level flight
• Low Mass
• low pitch
• low profile
• low quality
• low relief
• low spirits
• low St Andrew's cross
• low status
• Low Sunday
• low temperature
• low tide
• low water
• low-altitude
• low-backed
• low-beam
• low-birth-weight baby