恐らく（おそらく）

About

The adverb おそらく is similar in meaning to 多分たぶん and きっと, and means something along the lines of "probably" or "likely". おそらく is used when the likelihood of something happening/being the case is relatively high.

おそらく is fairly stiff in tone, and is most often used in negative scenarios (although not exclusively).

おそらく is commonly used in both its kanji and kana forms: おそらく・恐おそらく.

おそらく is most commonly used in formal written or polite spoken scenarios.

E.g. 恐おそらく、天候てんこうが原因げんいんで遅延ちえんしているのだと思おもいます。 "I think it is likely that the weather is the cause of the delay."

E.g. 日曜日にちようびだから、恐おそらくもう売うり切きれているでしょう。 "It's Sunday, so it's likely already sold out."

E.g. 猫ねこは一度いちど家出いえでをしたら、恐おそらく二度にどと戻もどってこない。 "Once a cat leaves the house, it's likely that it won't return."

Summary

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  おそらく is an adverb

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  おそらく is used to express "probably; likely"

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  おそらく is stiff in tone and is used in formal written or polite spoken scenarios