In (full) spate British phrase
This phrase literally means a river is full to overflowing with the strong flow of the water
Many rivers are in spate in the winter due to the heavy rainfall from their headwaters, causing severe flood in the areas downstream.
This phrase figuratively means completely engaging in talking that is fluent and not likely to stop or able to be interrupted.
Whenever seeing me, Lisa is in full spate about herself, which makes me feel fed up.
Originally Scottish and Northern English, of unknown origin, "a sudden flood, especially one caused by heavy rains or a snowmelt," Perhaps "flood" from Old French espoit, "flow, spout" from Dutch spuiten;" related to spout". A figurative sense of "unusual quantity" from the 1610s is attested.