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

See also: constant, energetic

• guava
• guava bush
• Guayaquil
• guayule
• gubbins
• gubernatorial
• guck
• gudgeon
• gudgeon pin
• guelder rose
• Guenevere
• guenon
• guenon monkey
• guerdon
• guereza
• gueridon
• guerilla
• guerilla force
• Guernsey
• guernsey elm
• guerrilla
• guerrilla force
• guerrilla theater
• guerrilla theatre
• guess