Definition of "understrapper"

I have ſeen him lately at Bullogne, and am perfectly vvell acquainted vvith ſome perſons vvho have ſupplied him vvith French lace and embroidery; and, as a proof of vvhat I allege, I deſire you vvill order him and this barber, vvho is his underſtrapper, to be examined on the ſpot.

1753, [Tobias Smollett], The Adventures of Ferdinand Count Fathom. […], volume I, London: […] W. Johnston, […], pages 214–215

No want of hands in our Theatre, Miss Bertram. No want of under strappers—My sister desires her love, and hopes to be admitted into the Company, and will be happy to take the part of any old duenna or tame Confidante, that you may not like to do yourselves.

1814 May 9, [Jane Austen], chapter XIII, in Mansfield Park: […], volume I, London: […] [George Sidney] for T[homas] Egerton, […], pages 268–269

The doctor immediately disappeared, returning soon after with a couple of flasks of wine concealed in the folds of his frock. Through the agency of the Marquesan, he had purchased them from an understrapper of the court.

1847 March 30, Herman Melville, chapter LXXXII, in Omoo: A Narrative of Adventures in the South Seas; […], London: John Murray, […]

A fine state of affairs, when a fifteen-dollar-a-week understrapper’s job gets to interfering with the business of an institution like the Wapatomac National Bank.

1927, Joseph C. Lincoln, chapter 3, in The Aristocratic Miss Brewster‎

My Lords, if an understrapper from a television company talks to an understrapper of the RAF, you do not expect air marshals to jump.

1977, Ian Winterbottom, Baron Winterbottom, Hansard, House of Lords, The Tornado military aircraft, 9 May, 1977

Alfie was familiar with the term ‘understrapper’. The Harry Fielding Alfie knew was an understrapper; a bob-a-job man for MI5. Somebody who could do a bit of house- or office-breaking, or some surveillance work.

2000, Philip Davison, McKenzie’s Friend, Penguin, page 37