Hast thou an arm like God? - The arm is the symbol of strength. The question here is, whether Job would venture to compare his strength with the omnipotence of God?
Or canst thou thunder with a voice like him? - Thunder is a symbol of the majesty of the Most High, and is often spoken of as the voice of God; see Psalm 29:1-11. The question here is, whether Job could presume to compare himself with the Almighty, whose voice was the thunder?

Hast thou an arm like God? - Every word, from this to the end of Job 40:14, has a wonderful tendency to humble the soul; and it is no wonder that at the conclusion of these sayings Job fell in the dust confounded, and ascribed righteousness to his Maker.

Hast thou an arm like God?.... Such power as he has, which is infinite, almighty, and uncontrollable, and therefore there is no contending with him; as he has an arm on which good men may lean on and trust in, and by which they are supported, protected, and saved, so he has an arm to crush like a moth all that strive with him or against him;
or canst thou thunder with a voice like him? thunder is his voice; see Job 37:4, &c. and is expressive of his power, Job 26:14; and his powerful voice may be observed in calling all things out of nothing into being in creation; in commanding and ordering all things in providence according to his pleasure; and in quickening sinners through his Gospel, by his Spirit and grace in conversion, and will be in calling men out of their graves and summoning them to judgment at the last day. God can both overpower and out voice men, and therefore it is in vain to oppose him and contend with him.

arm--God's omnipotence (Isaiah 53:1).
thunder--God's voice (Job 37:4).