A forked tongue
is a tongue
split into two distinct ends at the tip; this is a feature common to many species of reptiles
. Reptiles smell
using the tip of their tongue, and a forked tongue allows them to sense from which direction a smell is coming.
The image has given rise to the expression "to speak with a forked tongue", meaning to say one thing and mean another or, in more general terms, to act in a duplicitous manner.
Hummingbirds also have tongues that split at the tip. Galagos (bushbabies) have a secondary tongue, or sublingua, used for grooming, hidden under their first.