“The phrase “speaks with a forked tongue” means to deliberately say one thing and mean another or, to be hypocritical, or act in a duplicitous manner.” (Wikipedia)
James, an apostle, wrote: “If any of you lacks wisdom, let him ask God, who gives generously to all without reproach, and it will be given him. But let him ask in faith, with no doubting, for the one who doubts is like a wave of the sea that is driven and tossed by the wind. For that person must not suppose that he will receive anything from the Lord; he is a double-minded man, unstable in all his ways.” (1:5-8)
A double-minded man is unstable in all his ways…
Often the double-minded person will have a tongue that speaks out of both sides of their mouth. It takes a forked tongue to communicate from a double mind.
“From the same mouth come blessing and cursing. My brothers, these things ought not to be so.” (James 3:10)
“Draw near to God, and he will draw near to you. Cleanse your hands, you sinners, and purify your hearts, you double-minded.” (James 4:8)
“Hear and understand: it is not what goes into the mouth that defiles a person, but what comes out of the mouth; this defiles a person.” Jesus