Vocalizing one's opinions, beliefs, or ideologies in an impassioned, impromptu manner, often to others' annoyance. Soapboxes (crates in which soap was shipped) were once commonly used as makeshift platforms for such speeches. Once Grandpa got on his soapbox about the local election again, I found an excuse to slip out of the room. Whenever Jim is on his soapbox like this, I just try to tune him out.