Intentionally causing or reveling in pain and suffering; merciless, heartless.
The supervisor was very cruel to Josh, as he would always give Josh the hardest, most degrading work he could find.
(Can we date this quote?) Ranulph Fiennes, Cold: Extreme Adventures at the Lowest Temperatures on Earth
He was physically the toughest of us and wore five layers of polar clothing, but the cold was cruel and wore us down hour after hour.
(Can we date this quote?) C. S. Lewis, Prince Caspian: The Return to Narnia
You may be sure they watched the cliffs on their left eagerly for any sign of a break or any place where they could climb them; but those cliffs remained cruel.
(slang) Cool; awesome; neat.
be cruel to be kind
goodbye, cruel world
cruel (not comparable)
(nonstandard) To a great degree; terribly.
cruel (third-person singular simple presentcruels, present participlecruelling, simple past and past participlecruelled)
(chiefly Australia, New Zealand) To spoil or ruin (one's chance of success)
1937, Vance Palmer, Legend for Sanderson, Sydney: Angus & Robertson, p. 226, 
What cruelled him was that Imperial Hotel contract.
2014, The Sydney Morning Herald, 1 April, 2014, 
He was on the fringes of Test selection last year before a shoulder injury cruelled his chances.
2015, The Age, 8 September, 2015, 
A shortage of berth space for mega container ships will restrict capacity at Melbourne's port, cruelling Labor's attempts to get maximum value from its privatisation, a leading shipping expert has warned.
(Australia, transitive, intransitive) To violently provoke (a child) in the belief that this will make them more assertive.
2007, Stewart Motha, "Reconciliation as Domination" in Scott Veitch (ed.), Law and the Politics of Reconciliation, Routledge, 2016, p. 83, 
Violence is apparently introduced early by the practice of "cruelling": children even in their first months are physically punished and then encouraged to seek retribution by punishing the punisher.
2009, Mark Colvin, ABC, "Peter Sutton discusses the politics of suffering in Aboriginal communities," 2 July, 2009, 
[…] I was referring to the area where you were talking about this practice of cruelling; the pinching of babies, sometimes so hard that their skin breaks and may go septic.
cruel (countable and uncountable, pluralcruels)
Alternative form of crewel
cruel in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.
cruel in The Century Dictionary, The Century Co., New York, 1911
cruel (epicene, pluralcrueles)
From Old Occitan [Term?], from Latincrūdēlis.
cruel (masculine and feminine pluralcruels)
“cruel” in Diccionari de la llengua catalana, segona edició, Institut d’Estudis Catalans.
From Old Frenchcruel, from Latincrūdēlis; either remade based on the Latin or evolved from the Old French form crual, possibly from a Vulgar Latin form *crūdālis.