adj

• (now chiefly US) Dirty, filthy.
  "In such condition, there is no place for Industry; because the fruit thereof is uncertain: and consequently no Culture of the Earth; no Navigation, nor use of the commodities that may be imported by Sea; no commodious Building; no Instruments of moving, and removing such things as require much force; no Knowledge of the face of the Earth; no account of Time; no Arts; no Letters; no Society; and which is worst of all, continuall feare, and danger of violent death; And the life of man, solitary, poore, nasty, brutish, and short."
• (chiefly UK) Awkward, difficult to navigate; dangerous.
  "There was a nasty period during the First World War when the family's allegiance was called into question - not least because one of the Schroders had been made a baron by the Kaiser."
• (chiefly UK) Grave or dangerous (of an accident, illness etc.).
  "Moving into the middle ages, William the Conqueror managed to rout the English and rule the country, then see off numerous plots and assassination attempts, before his horse did for him in a nasty fall, killing him at 60."
• (slang, chiefly US) Formidable, terrific; wicked.
  ""Well," she grinned, looking hot and sexy. "They say you doing some real nasty shit in the NBA, but on the mic you a little too competitive. They say you a gaming niggah who likes to keep up all kinds of go-to-war gangsta friction between rappers. That shit must work for you though, huh? You stickin' all over the charts when other artists are failin' off. They rotating your cuts on the radio like mad too.""

noun

• (informal) Something nasty.
  "Processed foods are full of aspartame and other nasties."
• (euphemistic, slang, preceded by "the") Sexual intercourse.