1340, "having the same parents," derived from
german (1); cf.
human/humane, urban/urbane. Main modern sense of "closely connected, relevant" (1602) derives from use in "Hamlet" Act V, Scene ii: "The phrase would bee more Germaine to the matter: If we could carry Cannon by our sides," which is a fig. use of the word in the now-obsolete sense of "closely related, akin" (1470) in reference to things, not persons.