The late British author and newspaper columnist Simon Hoggart used to be fond of this word, applying it particularly to a governmental minion sent to make a statement before the House of Commons, thereby taking abuse that should have been directed at his boss. Few people now would grasp the full import of the word, though its first element gives enough of a clue that it’s an alternative to underling.
It was a popular term of the eighteenth century, its first user on record being Thomas Brown, a satirist now only remembered, if at all, for a verse translation of a Latin epigram he composed when under threat of being sent down by his Oxford tutor, Dr John Fell: “I do not love thee, Dr Fell, / The reason why I cannot tell; / But this I know, and know full well, / I do not love thee, Dr Fell.” In 1702, he wrote a book of mock letters from the recently deceased, including three supposedly from the late comic actor and satirical writer Joseph Haines to his friends at Will’s Coffee House:
I intend to build me a Stage in one of the largest Piazza’s of this city, take me a fine House, and set up my old Trade of Fortune-telling; and as I shall have upon occasion now and then for some Understrapper to draw teeth for me, or to be my Toad-eater upon the stage, if you will accept of so mean an Employment, beside my old Cloaths, which will be something, Ill give you Meat, Drink, Washing, and Lodging, and Four Marks per annum.
Letters from the Dead to the Living, by Thomas Brown, 1702. The wayward apostrophes are as printed.
It was still common during the nineteenth century:
“Dear me — very awkward!” said Stephen, rather en l’air, and confused with the kind of confusion that assails an understrapper when he has been enlarged by accident to the dimensions of a superior, and is somewhat rudely pared down to his original size.
A Pair of Blue Eyes, by Thomas Hardy, 1873.
My Collins Dictionary explains the second part as being from one sense of the verb strap, to work hard, which is also the source of the adjective strapping for someone big and strong (originally applied only to young women, by the way). A strapper could be a labourer or a man who groomed horses, hence a menial employee. His subordinate would be the ultimate underling.
If you would like an archaic alternative, try under-spur-leather, which is from the same area of life. A spur-leather was the strap that secured a spur to the rider’s foot, so somebody under the spur leather is figuratively beneath the heel of the rider.
There is a notorious Idiot, one hight Whachum, who from an under-spur-leather to the Law, is become an understrapper to the Play house, who has lately burlesqu’d the Metamorphoses of Ovid by a vile translation.
Remarks upon Mr Pope’s Translation of Homer, by John Dennis, 1717. Hight: named.
I offer it to Mr Hoggart for his consideration.