the.com/derailment
when the conversation, the train, or the argument leaves the tracks and finds it prefers the ditch.
means the sudden departure of something from its intended path, whether that something has wheels or was just a perfectly good point you were making.
from from french desrailler, off the rail — coined for trains in the 1830s, then borrowed by everyone whose meeting, thread, or therapy session also stopped going where it was supposed to.
psychology termcognitive derailment describes disordered, jumping thought patterns
internet lawany online thread eventually derails into arguing about semantics
deadliest causetrack defects cause more derailments than collisions do