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

See also: constant, energetic

• inebriant
• inebriate
• inebriated
• inebriation
• inebriety
• inedible
• ineducable
• ineffability
• ineffable
• ineffableness
• ineffably
• ineffaceable
• ineffaceably
• ineffective
• ineffectively
• ineffectiveness
• ineffectual
• ineffectuality
• ineffectually
• ineffectualness
• emplastic
• employ
• employable
• employe
• employed