From interior +‎ -ly.

interiorly (comparative more interiorly, superlative most interiorly)

1. In the interior part; internally; inwardly.

• exteriorly

• William Dwight Whitney, Benjamin E[li] Smith, editors (1911), “interiorly”, in The Century Dictionary[…], New York, N.Y.: The Century Co., →OCLC.
• “interiorly”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.