"of the same parents or grandparents," c. 1300, from Old French germain "own, full; born of the same mother and father; closely related" (12c.), from Latin germanus "full, own (of brothers and sisters); one's own brother; genuine, real, actual, true," related to germen (genitive germinis) "sprout, bud," which is of uncertain origin; perhaps it is a dissimilation of PIE *gen(e)-men-, suffixed form of root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups.
Your cousin-german (also first cousin) is the son or daughter of an uncle or aunt; your children and your first cousins are second cousins to one another; to you, your first cousin's children are first cousins once removed.
"a native of Germany," 1520s, from Latin Germanus (adjective and noun, plural Germani), first attested in writings of Julius Caesar, who used Germani to designate a group of tribes in northeastern Gaul, of unknown origin and considered to be neither Latin nor Germanic. Perhaps originally the name of an individual tribe, but Gaulish (Celtic) origins have been proposed, from words perhaps originally meaning "noisy" (compare Old Irish garim "to shout") or "neighbor" (compare Old Irish gair "neighbor"). Middle English had Germayns (plural, late 14c.), but only in the sense "ancient Teuton, member of the Germanic tribes." The earlier English word was Almain (early 14c., via French; see Alemanni) or Dutch. Shakespeare and Marlowe have Almain for "German; a German."
Þe empere passede from þe Grees to þe Frenschemen and to þe Germans, þat beeþ Almayns. [Ranulph Higden’s "Polychronicon," mid-14c., John Trevisa's translation, 1380s]
Their name for themselves, die Deutschen (see Dutch), dates from 12c. Roman writers also used Teutoni as a German tribal name, and writers in Latin after about 875 commonly refer to the German language as teutonicus (see Teutonic). Meaning "the German language" in English is from 1748. High German (1823 in English) and Low German as a division of dialects is geographical: High German (from 16c. established as the literary language) was the German spoken in the upland regions in southern Germany, Low German (often including Dutch, Frisian, Flemish), also called Plattdeutsch was spoken in the regions near the North Sea. In the U.S. German also was used of descendants of settlers from Germany.
"of or pertaining to Germany or the Germans," 1550s, from German (n.). German shepherd as a breed of dog (1922) is short for German shepherd dog (1889), which translates German deutscher Schäferhund. German Ocean as an old name for the North Sea translates Ptolemy. German measles attested by 1856. German-American is from 1880. German Reformed church is from 1812.