[Body Heat (April 1, 1981)]
Indicates unceasing activity, generally irritating to the speaker. Less commonly used as a term of admiration or respect. The victim of Body Heat upon realizing how screwed he was: Dont you understand? She was relentless! Its so relentless! used for relentless inanimate objects.
February 1983. Kissing the Pink Naked Big Man Restless – Hes so relentless
– Neal Manson (1985)