deep: reaching far down below the surface, whether measured with the eye or felt as something beyond reach
At its plainest this word means "deep": a surface that sinks below what surrounds it. Its most concrete use is the priest inspecting a skin sore to see whether it looks lower than the healthy skin around it (Leviticus 13), and it describes a pit dug deep enough to trap someone or a cup made deep enough to hold a great deal.
From there the word stretches, exactly as "deep" does in English, to name what is deep in the sense of unreachable: too far down to be measured or searched out. It is used of the human heart that no one else can see into (Psalm 64:6), of wisdom that runs deeper than the grave (Job 11:8), of truths too far down to be found (Ecclesiastes 7:24), and of the hidden things God brings up into the light (Job 12:22).
Because English "deep" does this same double duty, the word is easy to translate. What shifts with it is the tone, not the meaning. Depth can be a wonder, like words of counsel that never run dry, or a danger, like a pit you cannot climb out of or a schemer whose plans you cannot read. The word itself passes no judgment; the surrounding verse tells you whether the depth is inviting or threatening.