From historical +‎ -ly or historic +‎ -ally.

• (Received Pronunciation) IPA^(key): /hɪˈstɒɹɪkli/
• (General American) IPA^(key): /hɪˈstɔɹɪkli/

• Audio (US):

  (file)

historically (comparative more historically, superlative most historically)

1. In a historic manner; with reference to history or the historical record.

   Historically speaking, this company has always collected payment before starting work.

   Virtually all modern scholars of antiquity agree that Jesus existed historically.

2. According to history: formerly, in the past, traditionally.

   Historically, this company collected payment before starting work.

   • 2019 April 11, Harmeet Kaur, “Authorities say black metal may have influenced the Louisiana church fires suspect. Here’s what to know.”, in CNN‎^[1]:

     Holden Matthews, 21, has been charged in connection with fires at three historically black churches in Louisiana.

3. To an unprecedented or extremely rare degree.
   • 2018 February 23, Michelle Goldberg, “The De-Trumpification Agenda”, in The New York Times‎^[2], archived from the original on 23 February 2018:

     In January, the anti-corruption organization Citizens for Responsibility and Ethics in Washington, or CREW, released a detailed report on the historically unethical presidency of Donald Trump.

Historically serves as the adverb for both historic and historical. Thus, the distinction that is often made between these two adjectives (see the usage notes there) is lost for the adverb.