1With reference to a boat or ship: a horizontal or level keel, typically understood as something to be maintained in order to ensure smooth or stable sailing. Also in extended use with reference to maintaining a land vehicle or aircraft in a horizontal alignment.
2Figurative and in figurative contexts, with reference to holding a person or thing steady, or to restoring stability or normality after a difficult period. Especially in "on (also upon) an even keel" (and variants).
Early 17th century; earliest use found in John Smith. From even + keel.