1. Not to be placated or appeased or moved by entreaty
   "relentless persecution"
   - grim, inexorable, stern, unappeasable, unforgiving, unrelenting 
2. Never-ceasing
   "the relentless beat of the drums"
   - persistent, unrelenting

See also: continual, implacable, relentlessness

Encyclopedia: Relentless

• dead heat
• dead language
• dead letter
• dead load
• dead mail
• dead march
• dead metaphor
• dead nettle
• dead on target
• dead person
• dead reckoning
• dead ringer
• dead room
• Dead Sea
• Dead Sea scrolls
• dead set
• dead soul
• dead time
• dead to the world
• dead weight
• dead-air space
• dead-end
• dead-end street
• dead-man's float
• dead-man's-fingers