You sonuvabitch or You sonufabitch(pronounced son of a beech) is what Lily says when she gets really mad. Sometimes it's followed by her "You're dead to me look".
Notes and Trivia
Lily first said the phrase in The Playbook to Barney, after figuring out that he tricked her co-worker Shelly to sleep with him. And has occured on many episodes ever since.
Barney also said this phrase in the same episode
The phrase was first introduced by Lily in The Possimpible when Ted reminds Lily that she too has "weak-ass crap" on her resumé, pointing out that she includes her hot-dog eating contest on hers. Although you could argue this wasn't actually the pivotal point it become a "phrase" to the show. Note: this is also the first time we see Alyson Hannigan's pregnancy during season 4.
When talking to Marvin, Lily uses a second version of this phrase - You sonovame (son of a me).