cruel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Disposed to inflict pain or suffering.
• adjective Causing or characterized by severe pain, suffering, or distress.

from The Century Dictionary.

• Very; extremely.
• Disposed to inflict suffering, physical or mental; indifferent to or taking pleasure in the pain or distress of any sentient being; willing or pleased to torment, vex, or afflict; destitute of pity, compassion, or kindness; hard-hearted; pitiless.
• Proceeding from or exhibiting indifference to or pleasure in the suffering of others; causing pain, grief, or distress; performed or exerted in tormenting, vexing, or afflicting: as, a cruel act; a cruel disposition; the cruel treatment of animals.
• Synonyms Barbarous, savage, ferocious, brutal, merciless, unmerciful, pitiless, unfeeling, fell, ruthless, truculent, bloodthirsty, inexorable, unrelenting.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Disposed to give pain to others; willing or pleased to hurt, torment, or afflict; destitute of sympathetic kindness and pity; savage; inhuman; hard-hearted; merciless.
• adjective Causing, or fitted to cause, pain, grief, or misery.
• adjective Attended with cruetly; painful; harsh.
• noun See crewel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Not nice; mean; heartless.
• adjective slang Cool; awesome; neat.
• verb To spoil or ruin (one's chance of success)

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective (of persons or their actions) able or disposed to inflict pain or suffering