Germain means “Brother (in God)”

Germain Name Meaning

St. Germain (Germanus) led an outnumbered troop of British soldiers against an attacking army of Saxons and Picts on Easter day. Germanus ordered his troops to give the battle cry of “Alleluia,” and the cries were so terrifying to the Saxons that they fled or were drowned in the river, and the British won the battle without striking a blow.[5]
[and most Germains still cannot carry a tune]

French: A French family name made popular by a 5th-century saint, bishop of Auxerre.

It derives from Latin Germanus ‘brother’, ‘cousin’. In Italian it was enhanced to mean ‘brother (in God)’.

The feminine form, Germaine, is associated with a late 16th-century saint from Provençal, the daughter of a poor farmer, who was canonized in 1867.

Source: Dictionary of American Family Names ©2013, Oxford University Press