Arabic

maˀnat- - the entire part of the belly that surrounds the navel (BK 2 1054)

Vestiges of ­-ˀ- in Arb. rather point to *-ˀ-­ in the protoform; otherwise a biconsonantal root, with insertion of ­ ­-ˀ as a second radical in Arb. What is meant is probably rectus abdominis, a straight muscle of the abdomen.

PS
*mV(ˀ)n(-at)- - tendon, sinew; muscle (SED I No. 183)

Cf. *matn-  (1) ‘sinew, tendon, nerve’ (No. 191), related with metathesis?