This phrase translates to "I have a party in my pants." It is a playful and colloquial way to say that someone is in a good mood or feeling excited.
Defequé mis pantalones
Pantalones de vestir
¿Dónde compraste tus pantalones?
Tu bailas sin pantalones.
The big party would be "El fiesta grande" in Spanish.
In spanish, Bachelorette party translates to "despedida de soltera".
Tu fiesta
fiesta
Translation: Aqui no hay tal persona
el cerdo está en mis pantalones
Tengo un gato en mis pantalones.