This is an article about the Roman God in general. For Falacer in Nova Roma, see Falacer (Nova Roma).
Falacer is a God of uncertain origin. He was originally important enough to be warranted his own flamen (the Flamen Falacer), but by the late Republic he was highly neglected in public thought and his origins are lost to history.
Theories of Origins
There are a couple of theories of Falacer, none of them very convincing without more evidence. Some point to his title in Varro as a divus pater (Divine Father) as a reference to Iuppiter, although Bacchus is also called pater, especially with his Latin name Liber. Others point to the Roman author Festus who mentions that falandum is the Roman word for heaven, and thus would be a reference to the Heavenly God, or Iuppiter again. 
Although plausible, the Etruscan name for Iuppiter was Tinia, or thundering god, nor has either the name Falacer or falandum been found among Etruscan inscriptions which survive, though admittedly this is but an argument from silence.
The other theories are less solid linguistically. One points to a quasi-match of consonants with Hercules. H-F (not easily accepted, but not impossible), R-L (liquids were interchanged all the time), C-K (a perfect match), R-L (another liquid switch). The vowels, however, do not match up in any known way. The other points to a correlation with Falerii and the Falisci, and thus Falacer may have been the eponymous ancestor of these ancient tribes who mixed with the Romans since the Regal era.
Evidence in favor of this latter view is the further connection with Halaesus, the Greek companion of Agamemnon during the Trojan War who also went to Italy and fought against Aeneas. Halaesus was said to have founded the city of Falerii.