Doubly means twice as much, or in two ways at once. Doubly difficult, doubly sure, doubly unfortunate.
The second sense is the useful one: something doubly unfortunate is unfortunate for two separate reasons, and the word packs that into a syllable. Make doubly sure means check twice, which is one more time than people usually do.
1adverbdoublyTwice as much, or in two ways at once.
“Losing the match was doubly painful because it was against our old rivals.”
doubly is a degree.
Double plus -ly, from Latin duplus, twofold.
Middle English
dowbly
English, today
doubly
From Middle English dowbly, doubli, dubli, doubeliche, equivalent to double + -ly.
Etymology from Wiktionary, CC BY-SA