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

See also: constant, energetic

• gusher
• gushily
• gushing
• gushingly
• gushy
• gusset
• spring mattress
• spring onion
• spring peeper
• spring roll
• spring scale
• spring squill
• spring to mind
• spring training
• spring up
• spring vetch
• spring vetchling
• spring water
• spring-blooming
• spring-clean
• spring-cleaning
• spring-flowering
• spring-loaded
• springboard
• springbok