Truly means really and genuinely. A truly remarkable result, I am truly sorry, truly awful.
It works as an intensifier before an adjective, where it is stronger and more sincere than very, and it works as a comment on the whole sentence: truly, I had no idea. Note yours truly, the standard formal letter ending in British English when the recipient's name is not known, and its use as a joke meaning me. And the spelling is one of the reliable traps: true loses its e before -ly, unlike almost every other adjective of that shape.
1adverbtrulyReally and genuinely.
“It was a truly extraordinary piece of work.”
2adverbstressUsed to stress that what you are saying is sincere.
“I am truly sorry about the whole thing.”
Too few real examples of this word to estimate which sense is commonest.
truly is a degree of sincerity.
Old English
trēowlīċe — faithfully; truly
Middle English
truely
English, today
truly
From Middle English truely, treuly, treuli, trewely, treoweliche, treowliche, from Old English trēowlīċe (“faithfully; truly”), equivalent to true + -ly. Cognate with Dutch trouwelijk, Middle Low German truwlike, German treulich, Swedish trolig, Icelandic trygglega.
Etymology from Wiktionary, CC BY-SA