This is a classic expression that would show your DF friends you are on top of your Chilango slang. The expression te tiemblan las chichis (teh-tee-em-BLAN lass-CHEE-chees) literally means "your boobs shake." One's boobs shaking is a sign of fear. Thus, the expression means "you are afraid." It is typically used when challenging a buddy to misbehave. The challenged buddy is not likely to be afraid of the misbehaving itself but of the possible repercussions.
Carlos José: ¡Vamos al pelódromo! ¿O qué, te tiemblan las chichis?
Luis Augusto: Para nada. Vamos. Yo invito la primera ronda de privados.
Carlos José: Let's go to a strip club! Or, are you afraid?
Luis Augusto: Not at all. Let's go. I'll buy the first round of VIP private lap-dances.