Bear

• bear [countable]
• 1 a large strong animal with thick fur that eats flesh, fruit, and insects:
• a mother bear and her cubs
• ➔ grizzly bear, polar bear, teddy bear
• 2 American English informal something that is very difficult to do or to deal with:
• The chemistry test was a bear.
• 3

   be like a bear with a sore head

  British English informal to be rude to people because you are feeling bad-tempered
• 4 technical someone who sells shares or goods when they expect the price to fall [↪ bull]

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.