Plain English Meaning
The lowest spot in a tank or reservoir, where heavier stuff like water and dirt sinks and settles so it can be drained out before it causes trouble.
Full Definition
A low point in a fuel tank, oil system, or fluid reservoir where water, sediment, and other contaminants collect by gravity, and from which they can be drained through a sampling or drain valve.
Context Anchor
Seen during aircraft maintenance, preflight fuel checks, and inspections of fuel, oil, and drain systems.
Derivation
From Middle English 'sumpe', meaning a swamp or low marshy place where water collects. The aviation use keeps that idea: the lowest point where unwanted liquid gathers.