Definition of "thewy"

The rest of the crew, myself included, merely stood up to our spikes in heaving, whereas, unwontedly exhilarated, at every turn of the ponderous windlass, my belted comrade leaped atop of it, with might and main giving a downward, thewy, perpindicular heave, his raised eye bent in cheery animation upon the slowly receding shore.

1854, Herman Melville, The Encantadas