sound someone out Definition

sound³
verb, tr & intr

1. To measure the depth of (especially the sea).
2. medicine.
To examine (a hollow organ, etc) with a probe. See also sound¹ verb 5.

noun

1. A probe for examining hollow organs.

Etymology: 14c: from French sonder.

Phrasal Verb: sound someone or something out

To try to discover or to make a preliminary assessment of (opinions, intentions, etc).