Thanks for your question.
The Greek word is "meros" and means literally "thigh." However, we must remember that this book is a book of symbol and if we look at the use of "thigh" symbolically in scripture it is very instructive. In Abraham's day, a covenant was confirmed by the placing of the hand under the thigh of the one confirming the oath [see Genesis 24:2,9]. Jacob's thigh was damaged by the angel to teach him to rely on God and not on his own strength [see Genesis 32:25,31,32]. Psalm 45:2 depicts the conquering Christ with his sword upon his thigh. By this time in Revelation the "Lamb of God" is revealed as the "Lion of the tribe of Judah" riding forth to conquer, not in his own strength but by that of the God of Heaven. The covenant, once ratified by the blood of the lamb, will be sealed and fulfilled in the power of God as His son fills the earth with His glory.