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Webster 1913 Edition

adv.

In a manner not to be felt or perceived; imperceptibly; gradually.

The hills rise

insensibly

Addison.

adv.

Imperceptibly; in a manner not to be felt or perceived by the senses.

The hills rise insensibly.

By slow degrees; gradually. Men often slide insensibly into vicious habits.

insensibly ‎(comparative more insensibly, superlative most insensibly)