mid-14c., "having the same parents," derived from german
(1); cf. human/humane, urban/urbane
. Main modern sense of "closely connected, relevant" (c.1600) 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" (late 15c.) in reference to things, not persons.