outfall
A sudden eruption of troops from a fortified place; sally.
obsolete
A quarrel; a falling out.
The point or place of discharge of a river, drain, culvert, sewer, etc.; mouth; embouchure.
West of Keswick a short descent at 1 in 122 brings the train down to the low-lying and marshy ground between Derwentwater and Bassenthwaite Lake and to the crossing of the Derwent - the outfall from Derwentwater, [...].
Where it comes from
From out- + fall. Compare Dutch uitval (“outburst, sally, eruption”), German Ausfall (“falling out, sally”), Swedish utfall (“sally, issue”).
Adapted from the Wiktionary entry outfall (CC BY-SA 4.0).