cross-beam above or under a gate
楣 centers on a horizontal structural beam placed above or below a gate, serving as a lintel or threshold support.
楣 combines 木 (wood) with 眉, which likely contributes the sound and may relate to the brow-like position of a lintel above an opening. The exact historical development is uncertain.
The left side 木 is wood, and the right side 眉 suggests an eyebrow. Picture a wooden beam sitting like an eyebrow above a gate, forming the cross-beam.
For ビ, imagine a bee buzzing under the wooden beam above the gate: bee -> ビ, and the bee rests on the cross-beam.
lintel (lacking the grooves needed for a sliding door)