Definitions
mahobi: The hollow spaces below projecting eyebrows (e.g. of baboon)
mubvumbi: Thin shower
Translation
If you see a baboon soaked beneath its heavy brow ridge, know that the drizzle has been falling for a long time.
Application
This proverb teaches that small signs often reveal larger truths. If something or someone typically resistant or protected shows signs of strain or change, then the situation is likely deeper or older than it appears.