tubnu -tu'u- gismu

x₁ is a length of tubing/pipe/hollow cylinder [shape/form] of material x₂, hollow of material x₃.

Also tube, sleeve, leg, hose, (adjective:) tubular. See also kevna, canlu.

In notes:

canlu: x₁ is space/volume/region/room [at-least-3-dimensional area] occupied by x₂.
kevna: x₁ is a cavity/hole/hollow/cavern in x₂; x₁ is concave within x₂; x₂ is hollow at locus x₁.
damtu'u: t₁ is a chimney made of of material t₂ for emitting/expelling smoke/gas/steam/magma t₃=d₁ from source d₂.
damtubnu: t₁ is a chimney made of of material t₂ for emitting/expelling smoke/gas/steam/magma t₃=d₁ from source d₂.
moltu'u: t₁ is a mouthpiece/embouchure of material t₂ conveying fluid t₃.
tu'urbirka: b₁= t₁ is a tentacle / tube-shaped arm [body part] of b₂.