The word pegasteis is the preterite form of pegar in the second person plural.
See the full pegar conjugation
Después de cómo os pegasteis el otro día, no lo creo.
After the way you guys were beating each other up the other day, I don' think so.
¿Por qué le pegasteis?
Why'd you kick him out?
Tengo que decirte que, nunca me pegásteis, los dos.
You know, I got to tell you, never made sense to me, the two of you.
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