A formal, somewhat literary adverb used to express a strong wish or hope, often at the beginning of a sentence. Equivalent to 願わくは.
See also: 願わくは
I pray that this plan will succeed.
I sincerely hope for your cooperation.
Alternate kanji spelling using 希; less common than 願わくば.
From classical 願わくは. 願わく is a nominalized form of 願う formed by the classical ク語法 construction, and は is the particle; the variant 願わくば arose through voicing of the particle.