An ARM — and idiomatically, power exerted. 'The arm of Jehovah' opens Isaiah 53 as a question: to whom has it been revealed? The idiom runs through the Exodus story ('an outstretched arm', Deuteronomy 4:34), so the chapter begins by asking whether anyone recognised God's power when it appeared — and the answer the poem gives is that it appeared in a form nobody would look at twice. → first discussed at Isaiah 53:1