WordNet sense 1 (very strong or vigorous):

hard, knockout

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

strong

WordNet sense 3 (severely simple):

stark, stern, austere

WordNet sense 4 (intensely or extremely bad or unpleasant in degree or quality):

terrible, wicked, wicked

WordNet sense 5 (in an extreme degree):

intense

WordNet sense 6 (unsparing and uncompromising in discipline or judgment):

spartan

WordNet sense 7 (characterized by strictness, severity, or restraint):

strict, nonindulgent

WordNet sense 8 (having undesirable or negative qualities):

bad

WordNet sense 9 (causing fear or anxiety by threatening great harm):

dangerous, grievous, life-threatening, serious, grave

WordNet sense 10 (being in or verging on a state of crisis or emergency):

critical