Verily means truly, in old or biblical use. Verily I say unto you, verily it was so, verily and indeed.
It belongs to the King James Bible above all, and anybody using it now is either quoting or joking. It is built on very in its older sense of true, which survives in the very man and in verity, meaning truth.
1adverbverilyTruly, in old or biblical use.
“In the King James Bible, Jesus often begins a teaching with "Verily I say unto you."”
verily is a manner.
Middle English verraily, from verray, true, from Latin verus. Very is the same word.
Middle English
verraily
English, today
verily
Built from
From Middle English verraily, from verray (“true, very”) + -ly. More at very, -ly.
Etymology from Wiktionary, CC BY-SA