The word mentirías is the conditional form of mentir in the second person singular. There are other translations for this conjugation.
See the full mentir conjugation

mentir

La pregunta es... ¿tú mentirías por tu papá?
The question is... would you lie for your dad?
¿Tú no mentirías para obtener un trabajo como éste?
Wouldn't you lie to get a job like this?
Bueno, no le mentirías a los Estados Unidos, ¿eh, Bart?
Well, you wouldn't lie to the United States, would you, Bart?
¿Qué te hace pensar que no mentirías también a todos?
What makes you think you wouldn't lie to everyone there, too?
Tú no me fingirías a mí, no me mentirías así.
You wouldn't pretend to me, you wouldn't lie like that.
Nunca pude entender por qué tú me mentirías deliberadamente.
I could never understand why you would deliberately lie to me.
¿Por qué mentirías a tus amigos y les dirías que todo estaba bien?
Why'd you lie to your friends and tell them everything was fine?
¿Por qué le mentirías a tu esposa?
Why would you lie to your wife?
¿Por qué le mentirías a la policía?
Why would you lie to the cops?
Sé que nunca me mentirías, Lily.
I know that you'd never lie to me, Lily.
¿Por qué mentirías a tus amigos y les dirías que todo estaba bien?
Why'd you lie to your friends and tell them everything was fine?
Papi, me dijiste que jamás me mentirías.
Papi, you told me you'd never lie to me.
El otro día me dijiste que nunca me mentirías
You told me the other day that you'd never lie to me.
No le mentirías a un policía, ¿no?
You wouldn't lie to a policeman, would you?
Claro, y seguro no nos mentirías.
Right, and I'm sure you wouldn't lie to us.
Tú no mentirías sobre la salud del detective Paul.
You shouldn't have lied about Detective Paul's health.
Porque tú nunca me mentirías, ¿cierto?
Because you'd never lie to me, right?
Sé que no mentirías por él.
I know you wouldn't lie for him.
Sabemos que no le mentirías a ella.
Oh, we know you don't lie to her.
¿Por qué nos mentirías a todos?
Why would you lie to all of us?
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