cutthroat

Short definition from Wiktionary

Noun

1. 1
   A murderer who slits the throats of victims.
2. 2
   An unscrupulous, ruthless or unethical person.
3. 3
   A three-player pocket billiards game where the object is to be the last player with at least one ball still on the table.
4. 4
   Ellipsis of cutthroat compound (“an agentive-instrumental verb-noun compound word”).

Adjective

1. 1
   Involving the cutting of throats.
2. 2
   Of or relating to a card game where everyone plays for him or herself rather than playing with a partner.
3. 3
   Ruthlessly competitive, dog-eat-dog.