(idiomatic,euphemistic,of a man) To **** or to ejaculate when the **** has been withdrawn from one's partner.
1609, Holy Bible, Douay Version, "Genesis" 38:9:
He knowing that the children should not be his, when he went in to his brother's wife, spilled his seed upon the ground, lest children should be born in his brother's name.
2003, Jon McGregor, If Nobody Speaks of Remarkable Things, Houghton Mifflin, ISBN 9780618344581, p. 224:
She thinks of the shame of her husband, having to spill his seed into a plastic bottle.