David is referring to his enemies here, and people who rejected God’s authority, and spells out some attributes of wicked people, 

Psa 5:9  There is nothing reliable in what they say; Their inward part is destruction itself. Their throat is an open grave; They flatter with their tongue.

Wicked folks are not reliable, they have wicked hearts, talk lots of smack but can also say all the right things.  

Now we can hop on here and think of all the different people we know that aren’t reliable and are brown-nosers, like David’s enemies were.  

However, we should also take an inward look out ourselves on this matter too.  If we put ourselves under the scrutiny of the Bible, chances are we have fallen short in these areas too.  Not sure about you, but there have been times when I haven’t done what I said I’d…I have also harbored hate for my enemies, talked smack, and told the boss what they wanted to here.  Chances are, you probably have too…lol.  

These are just part of our human nature that we inherited from Adam when He ate the forbidden fruit, just like most of us are born with 10 toes, we are also born with this wicked nature that seems to love sin.  

This also just underscores how much we need The Gospel in our lives, and God’s Word here that scrutinizes our own way of life, shows us our sin, and shows us how Christ takes that sin away, because we can’t do it on our own, we must lean upon The Almighty through studying The Word, praying, and repenting or putting aside our old evil ways.