To plunge, dip, or immerse something beneath the surface of a liquid.
From Latin demergere: de- (down) + mergere (to dip or plunge). The root mergere also gave us 'merge' and 'submerge,' all sharing the idea of liquid submersion.
This is a rare gem in English—most people use 'immerse' or 'submerge,' but 'demerse' preserves an older, more direct Latin path that feels almost more honest about what's happening: you're pushing something down into water.
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