Definition of "unconquerable"

"Be careful when power comes to thee also, lest thou too shouldst smite in thine anger or thy jealousy, for unconquerable strength is a sore weapon in the hands of erring man."

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887