This is one of those expressions that might overwhelm the listener because of its imagery. Cagarse para adentro (cah-GAR-seh pa-rah DEN-trow) or cargarse pa'dentro literally means to defecate inwards, in the direction opposite to what nature intended. Even though it makes no biological sense, cagarse para adentro denotes quite an abnormal body reaction to a situation, an event or some news. Cagarse used alone is not that dramatic.
Rogelio: ¿Cómo se tomó tu jefe la renuncia en esta época del año?
César: Se cagó pa'dentro. Están hasta las nalgas de chamba.
Rogelio: How did your boss take your resignation this time of the year?
Cesar: It was a big blow for him. They are swamped with work.