Definition of "thrumming"

Those unfortunate and well-educated women made themselves heard from the neighbouring drawing-room, where they were thrumming away, with hard fingers, an elaborate music-piece on the piano-forte, as their mother spoke; and indeed, they were at music, or at backboard, or at geography, or at history, the whole day long.

1847 January – 1848 July, William Makepeace Thackeray, chapter 20, in Vanity Fair […], London: Bradbury and Evans […], published 1848

The harp itself was a huge gourd, and a most unmusical "shell" it proved to be. It had three strings, the thrummings of which disquieted me on two accounts […]

1870, The African Repository and Colonial Journal, volume 46, page 231