what is the source of the term lake "soundings"?

Answer 1

It's called this because the measurements are taken using sound waves. A sound pulse is sent down into the water, and the time for the echo to be received is measured. Since the speed of sound in water is known, we can divide half this time (half, because the pulse must reach the bottom and return the same distance) by the speed of sound to get the depth.

Answer 2

sonar is sound waves under water to determine depth,soundings is a reference to that