Fruit of your loins
From the Bible, Genesis 35:11 (King James Version):
And God said unto him, I am God Almighty: be fruitful and multiply; a nation and a company of nations shall be of thee, and kings shall come out of thy loins;
Should you be asked to 'gird up your loins', or otherwise wonder where your loins are exactly, you could refer to this rather coy definition from the OED:
[Loins]: "The part of the body that should be covered by clothing."
That is what might have been called in the UK throughout most of the 20th century as 'the nether regions', and more recently 'the wobbly bits'.