Is it ensure, insure, or assure?
early 15c., "a prompting," from Latin instinctus "instigation, impulse," noun use of past participle of instinguere "to incite, impel," from in- "on" (see in- (2)) + stinguere "prick, goad," from PIE *steig- "to prick, stick, pierce" (see stick (v.)). Meaning "animal faculty of intuitive perception" is from mid-15c., from notion of "natural prompting." Sense of "innate tendency" is first recorded 1560s.
instinct in·stinct (ĭn'stĭngkt')
An inborn pattern of behavior that is characteristic of a species and is often a response to specific environmental stimuli.
A powerful motivation or impulse.
An inherited tendency of an organism to behave in a certain way, usually in reaction to its environment and for the purpose of fulfilling a specific need. The development and performance of instinctive behavior does not depend upon the specific details of an individual's learning experiences. Instead, instinctive behavior develops in the same way for all individuals of the same species or of the same sex of a species. For example, birds will build the form of nest typical of their species although they may never have seen such a nest being built before. Some butterfly species undertake long migrations to wintering grounds that they have never seen. Behavior in animals often reflects the influence of a combination of instinct and learning. The basic song pattern of many bird species is inherited, but it is often refined by learning from other members of the species. Dogs that naturally seek to gather animals such as sheep or cattle into a group are said to have a herding instinct, but the effective use of this instinct by the dog also requires learning on the dog's part. Instinct, as opposed to reflex, is usually used of inherited behavior patterns that are more complex or sometimes involve a degree of interaction with learning processes.
Behavior that is not learned but passed between generations by heredity.