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

See also: constant, energetic

• stave in
• stave off
• stave wood
• stavesacre
• stay
• stay at
• stay away
• stay fresh
• stay in place
• stay of execution
• stay off
• stay on
• stay over
• stay put
• stay together
• stay up
• stay-at-home
• stayer
• staying power
• Stayman
• Stayman Winesap
• stays
• staysail
• STD
• stead