Bad  —adj. (worse, worst) 1 inadequate, defective (bad work, light). 2 unpleasant (bad weather). 3 harmful (is bad for you). 4 (of food) decayed. 5 colloq. Ill, injured (feeling bad today; a bad leg). 6 colloq. Regretful, guilty (feels bad about it). 7 serious, severe (a bad headache, mistake). 8 a morally unacceptable (bad man; bad language). B naughty. 9 not valid (a bad cheque). 10 (badder, baddest) esp. Us slang excellent. —n. Ill fortune; ruin. —adv. Us colloq. Badly.  not (or not so) bad colloq. Fairly good. Too bad colloq. Regrettable. [old english]