To chug is a recent example of back-formation, since it derives from chugger which was coined before it:
chugger, someone who is paid to stop people in the street and persuade them to give money regularly to a Charity → to chug, to do just that. Many people don't like to be 'chugged', as they consider that, in a way, they're being 'mugged' ('atracados'), making them waste their time when maybe they're in a hurry, etc.
Other examples of back-formation:
burglar (ladrón que roba en casas/tiendas, etc) → to burgle (robar en casas/tiendas, etc)
diagnosis (diagnóstico) → to diagnose (diagnosticar)
peddler (vendedor,-a ambulante) → to peddle (vender de puerta en puerta)
sculptor (escultor,-a) → to sculpt (esculpir)
sightseeing (visita turística) → to sightsee (realizar una visita turística)television (televisión) → to televise (televisar)