Definition from Wiktionary, the free dictionary
From Latin speciōsus (“good-looking”).
- Seemingly well-reasoned or factual, but actually fallacious or insincere; strongly held but false.
- This idea that we must see through what we have started is specious, however good it may sound.
- 1776, Thomas Paine, Common Sense
- I have frequently amused myself both in public and private companies, with silently remarking, the specious errors of those who speak without reflecting.
- Having an attractive appearance intended to generate a favorable response; deceptively attractive.
- (obsolete) Beautiful, pleasing to look at.
- (seemingly well-reasoned, but fallacious): fallacious, insincere
- (with appearance intended to generate a favorable response): meretricious
Derived terms 
seemingly well-reasoned, but fallacious