Green’s Dictionary of Slang

a favour that is granted in return for one favour received, repaying one positive gesture with another; the phr. particularly refers to an exchange of congratulatory toasts.

1662		Fuller Worthies II (1840) 40: This is generally taken in a good and grateful sense, for the mutual return of favours received. [...] However, sometimes Hertfordshire kindness may prove to be Hertfordshire cruelty.
c.1698		B.E. Dict. Canting Crew n.p.: Hartfordshire-kindness, Drinking to the same Man again.
1701		N. Ward ‘A Walk to Islington’ in Writings (1704) 75: For want of a Third in our Mess, we were fain, / To use Hartfordshire kindness, Here’s to you again.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1738		Swift Polite Conversation 60: nev.: My Lord; this Moment, I did my self the Honour to drink to your Lordship. lord sm.: Why then, that’s Hartfordshire Kindness.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Hertfordshire kindness, drinking twice to the same person.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1878		Luton Times 12 July 7/5: ‘Hertfordshire kindness’ (may prove ‘Hertfordshire cruelty’).