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