river source; headwaters
Refers to the geographical origin of a river or stream.
The source of that river is deep in the mountains.
Metaphorical extension of the river-source meaning; used for the fundamental cause or origin of something.
It is important to find the root cause of the problem.
Compound of 源 (gen, 'source') and 頭 (tō, 'head'), literally 'source head'. The metaphorical sense follows naturally from the geographical meaning.