Sprog is a British
aspersion for a child, akin to the American term "
runt", "
ankle biter" or "
rug rat". Sprog in all likelihood comes from the term "
sprag" which refers to a young
salmon. In the 18th century, sprag came to refer to a
gay, cheery youth.
By
World War II, "sprog" had entered into the British army's slang
lexicon and was used in a
derogatory sense to refer to a new recruit or someone in the lower ranks. After the war, former soldiers started making lots and lots and lots of babies and sprog, being too good a word to forget, got applied to children.