The first occurrence.
And he lifted up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground, … Gen 18:2
(“Forever, O LORD, Your word stands (H5324) in heaven.” Psa 119:89)
The primitive root.
Strong’s H5324 נצב natsab, a primitive root, “to stand.” The 3-letter root is nun + tsadey + bet.
nun ן, נ = the seed, thus continue, heir, son
tsadey ץ, צ = the trail, thus a man concealed, journey, chase, hunt
bet ב = the house, thus house, household, family, in, within
When a man conceals himself while hunting, he is often on his side. So the side (tsadey) of the house (bet) is a wall, one of the cognate words from this root. The story: An erect (nun) wall (tsadey + bet), i.e. standing firm as an erect wall.