different between sulky vs relentless

English

From sulk +? -y. The horse-drawn vehicle is so called as it obliges the rider to be alone.

sulky (comparative sulkier, superlative sulkiest)

(often derogatory) silent and withdrawn after being upset
- 1865, Lewis Carroll, Alice's Adventures in Wonderland
  The first question of course was, how to get dry again: they had a consultation about this, and after a few minutes it seemed quite natural to Alice to find herself talking familiarly with them, as if she had known them all her life. Indeed, she had quite a long argument with the Lory, who at last turned sulky, and would only say, “I’m older than you, and must know better.” And this Alice would not allow, without knowing how old it was, and, as the Lory positively refused to tell its age, there was no more to be said.
Synonyms: sullen, morose

sulky (plural sulkies)

relent +? -less

relentless (comparative more relentless, superlative most relentless)

Unrelenting or unyielding in severity.
Unremitting, steady and persistent.
- 2016 January 31, "Is Huma Abedin Hillary Clinton’s Secret Weapon or Her Next Big Problem?," Vanity Fair (retrieved 21 January 2016):
  In Clintonworld, the reaction to Grassley’s relentless assault on Huma is one of resignation.

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