Noun

1. A murderer who slits the throats of his victims.
2. An unscrupulous, ruthless or unethical person.

Adjective

1. Involving the cutting of throats
2. Of or relating to a card game where everyone plays for him or herself rather than playing with a partner.

   He found that playing cutthroat Spades was much more difficult than playing with a partner.

3. Ruthlessly competitive, dog-eat-dog

   Law is a cutthroat business, you always have to look out to see who is trying to outdo you.