恐らく
おそらく · osoraku · JLPT N3 · adverb
perhaps
恐 = fear: from osoru (to fear) + -aku, so it means I fear that... → perhaps
Osoraku comes from the old verb osoru, to fear, plus -aku, which turns the verb into a noun: what one fears is... It softened into probably or perhaps. Sound hook: oh, so rocky? Perhaps.