Source: Make Me a Hanzi.
沉 chén shows water 氵 with 冗 yín giving the sound; here 冗 stands for a variant of 冘. It first meant lowering cows or sheep into a river as an offering to the water god, so the god could take them. From that came 'to submerge' and 'to sink', then 'heavy', 'to hide' and 'deep'.
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