WordNet sense 1 (very strong or vigorous):

knockout, severe

WordNet sense 2 (having strength or power greater than average or expected):

strong

WordNet sense 3 (produced without vibration of the vocal cords):

unvoiced, voiceless, surd

WordNet sense 4 (given to excessive indulgence of bodily appetites especially for intoxicating liquors):

heavy, intemperate

WordNet sense 5 (characterized by or given to yielding to the wishes of someone ):

indulgent

WordNet sense 6 ((of light) transmitted directly from a pointed light source):

concentrated

WordNet sense 7 (resembling steel as in hardness):

steely

WordNet sense 8 (resistant to physical force or pressure):

unyielding

WordNet sense 9 (converted to solid form (as concrete)):

set

WordNet sense 10 (hard as granite):

stony

WordNet sense 11 (not soft or yielding to pressure):

firm, solid

WordNet sense 12 (having the hardness of a diamond):

adamantine

WordNet sense 13 (having undesirable or negative qualities):

bad

WordNet sense 14 (characterized by toilsome effort to the point of exhaustion; especially physical effort):

backbreaking, grueling, gruelling, laborious, punishing, toilsome, operose

WordNet sense 15 (hard to endure):

trying

WordNet sense 16 (difficult to deal with):

troublesome

WordNet sense 17 (requiring effort or concentration; complex and not easy to answer or solve):

serious

WordNet sense 18 (very difficult; severely testing stamina or resolution):

rugged, tough, rugged

WordNet sense 19 (full of hardship or trials):

rocky, rough

WordNet sense 20 (exasperatingly difficult to handle or circumvent):

tight

WordNet sense 21 (making great mental demands; hard to comprehend or solve or believe):

baffling, knotty, problematic, problematical

WordNet sense 22 (hard to deal with; especially causing pain or embarrassment):

sticky

WordNet sense 23 (difficult to accomplish; demanding considerable mental effort and skill):

arduous

WordNet sense 24 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult

WordNet sense 25 (indulging excessively):

heavily, intemperately

WordNet sense 26 (with firmness):

firmly

WordNet sense 27 (causing great damage or hardship):

severely