"Impregnable" and "impregnate" seemingly opposites? [duplicate]

Solution 1:

They just have different etymologies: prenable vs praegnare

Impregnable (adj.):

• early 15c., imprenable "impossible to capture," from Middle French imprenable "invulnerable," from assimilated form of in- "not, opposite of" (see in- (1)) + Old French prenable "assailable, vulnerable" (see pregnable).

Impregnate:

• 1600, from Late Latin impraegnatus "pregnant," past participle of impraegnare "to render pregnant," from assimilated form of in- "into, in" (see in- (2)) + praegnare "make pregnant"

(Etymonline)

Solution 2:

Here, the 1 and 2 meanings of "im-" are completely spelled out with examples.

http://dictionary.reference.com/browse/im-

Note that (in both cases) "im-" and "in-" are just two variant spellings of the same thing; there are any number of word examples of all four possibilities.