originally from Middle French adoulcir, also written addoulcir; (mod. adoucir) to sweeten:—late Latin addulcīre; from ad to + dulcis sweet. Subsequently refashioned after Latin.
From: The Compleat Ambassador:
Or Tow Treaties of the Intended Marriage of Qu: Elizabeth, Of Glorious Memory
Collected by Sir Dudly Diggs, Printed 1655
The Queens Majesties Letter to the French King