Definition of "plaguily"

[…] most wicked woman (that hast so plaguily a corrupted minde, as thou canst not keepe thy sickenesse to thy selfe, but must most wickedly infect others) […]

c. 1580 (date written), Philippe Sidnei [i.e., Philip Sidney], “[The Second Booke] Chapter 10”, in Fulke Greville, Matthew Gwinne, and John Florio, editors, The Countesse of Pembrokes Arcadia [The New Arcadia], London: […] [John Windet] for William Ponsonbie, published 1590; republished in Albert Feuillerat, editor, The Countesse of Pembrokes Arcadia (Cambridge English Classics: The Complete Works of Sir Philip Sidney; I), Cambridge, Cambridgeshire: University Press, 1912, page 284

I was plaguily nettled, and disappointed too.

1748, [Samuel Richardson], “Letter XLVI”, in Clarissa. Or, The History of a Young Lady: […], volume V, London: […] S[amuel] Richardson; […], page 318

Little wonder that the artist’s glance frequently wandered across the great shining table towards a girl who, if they had not been so plaguily intent on honouring his fame, might have now been replacing the Mayoress at his side.

1907, Israel Zangwill, “The Jewish Trinity”, in Ghetto Comedies‎, London: Heinemann, page 89