Definition of "doggedly"

I grew moody, silent, and unsocial, but studied on doggedly and incessantly.

1820, Washington Irving, “The Early Experiences of Ralph Ringwood”, in The Crayon Papers

I continued to pound along doggedly. I was grimly resolute.

1906, P. G. Wodehouse, chapter 6, in Love Among the Chickens

She was married to someone.He was doggedly determined he would get her.

1983, Paul Simon, Train in the Distance

Unemployment hovered doggedly near 10%.

2010 December 9, Ishaan Tharoor, “Obama's Quagmire II: The Economy”, in Time, retrieved 28 April 2014

'Nay,' said Dr Johnson, 'a man may write at any time, if he will set himself DOGGEDLY to it.' [Footnote: This word is commonly used to signify sullenly, gloomily and in that sense alone it appears in Dr Johnson's Dictionary. I suppose he meant by it 'with an OBSTINATE RESOLUTION, similar to that of a sullen man'.]

1785, James Boswell, The Journal of a Tour to the Hebrides with Samuel Johnson, LL.D.