A crazy guess:
Germa:n- may be a
straightforward Latin translation of Germanic Gut-an- 'Goths'.
For a linguistic analysis of the latter term, see
If the weak-declension stem that serves as
name of the Goths is derived from the verb root represented by Gothic
giutan 'flow', -gutnian 'pour', it may have
meant something like 'springing forth (from the same source), offshoot', hence
the Latin calque germa:nus 'of the same race, having the same
parents' (cf. germen 'embryo, sprout, offshoot').
Opinions?
Piotr