birka -bir- gismu

x₁ is a/the arm [body-part] of x₂; [metaphor: branch with strength].

Also elbow (= bircidni), wrist (= xanterjo'e), appendage (but jimca, rebla preferred). See also jimca, janco, xance, rebla.

In definition:

jimca: x₁ is a branch/bough/limb of x₂; x₂ forks into branches x₁; [preferred over metaphorical birka].

janco: x₁ is a/the shoulder/hip/joint [body-part] attaching limb/extremity x₂ to body x₃.
rebla: x₁ is a/the tail/appendix [body-part] of x₂; [metaphor: trailing, following portion/appendage].
xance: x₁ is a/the hand [body-part] of x₂; [metaphor: manipulating tool, waldo].
pecai (comp!): restrictive relative phrase marker: permanently / inalienably associated with; it is impossible or near-impossible to break that relation
birgutci: g₁ is g₂ cubit/cubits (length unit).
birja'i: j₁ is a bracelet on arm j₂=b₁.
birjunla: j₁ is a wristwatch measuring time units j₂ to precision j₃ with timing mechanism/method j₄.
birstizu: s₁ is an armchair.
galbirka: g₁=b₁ is a forearm of b₂.
mumbircurnu: x₁ is a starfish of species x₂.
selbirka: x₂=b₁ is an arm of body x₁=b₂.
tu'urbirka: b₁= t₁ is a tentacle / tube-shaped arm [body part] of b₂.
xancidni: x₁=c₁ is a wrist of x₂=c₂.
tubra: x₁ is a limb/appendage/extremity of body/entity x₂ used for purpose x₃
utka: x₁ and x₂ are path-linked by binary predicate x₃ (ka) via intermediate steps x₄ (ce'o; (ordered) list).