From insensible + -ly.

insensibly

1. In a insensible manner; without feeling or noticing something.
   • 1705, J[oseph] Addison, “From Rome to Naples”, in Remarks on Several Parts of Italy, &c. in the Years 1701, 1702, 1703, London: […] Jacob Tonson, […], →OCLC ↗, page 174 ↗:

     St. Peters ſeldom anſvvers Expectation at firſt entering it, but enlarges it ſelf on all Sides inſenſibly, and mends upon the Eye every Moment.