perfidious
1. [ adjective ] tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans
Synonyms: treacherous punic
Examples:

"Punic faith" "the perfidious Judas" "the fiercest and most treacherous of foes" "treacherous intrigues"

Related terms: unfaithful
Similar spelling:   perfidiously
  perfidy