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.
damtubnu
t1 is a chimney made of of material t2 for emitting/expelling smoke/gas/steam/magma t3=d1 from source d2.
damtu'u
t1 is a chimney made of of material t2 for emitting/expelling smoke/gas/steam/magma t3=d1 from source d2.
fletu'u
x1=t1 is a pipe/tube/hose carrying fluid x2=f2=t3 towards x3=f3 from x4=f4 (direction/source)
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.
tu'urpi'o
p1 is an organ/pipe organ/mechanical organ (keyboard musical instrument).
torxesrtubnu
x1 is a tube/hole/tunnel through (topological) torus x2.