Breakfast with Solomon - Proverbs 25:19
"Like a bad tooth and an unsteady foot is confidence in a faithless man in time of trouble"
The word faithless is the word often translated deceit or treachery, unfaithfully. The person who says one thing all the while planning or plotting another. The person who always has a plan B that will save their skin at the expense of someone else.
There is a natural tendency to trust them because they seem so sincere and can tell you how much they plan on carrying through with this plan or arrangement; but if they have dealt treacherously or deceitfully with others before or even you before, then do not put a lot of stock in their coming through.
This often happens in situations where relatives are being counted on to come through when every indication of the past is that they will not come through. They will again be treacherous this time even though each time they don't mean to be late or leave you in the lurch – they just always do.
There are some leaders who always and only think of themselves and because of that they are treacherous. They always will put someone else at risk and never themselves.