John Gill Commentary Psalms 78:18

"And they tempted God in their heart By asking food according to their desire." — Psalms 78:18 (ASV)

And they tempted God in their heart
Which is desperately wicked, and from which all impiety flows; they entertained hard thoughts of God; they inwardly fretted at their present circumstances, and secretly repined and murmured against God, and wished for things they should not; not being contented with what they had, and thankful for them, as they ought to have been:

by asking meat for their lust ;
or, "for their soul"; such as their souls lusted after, and their sensitive appetites craved; for they were not satisfied with the bread they had, which was sufficient for their sustenance and support; they wanted food for pleasure and wantonness; to ask for daily bread is right, but to ask for more to consume on our lusts is wrong, (James 4:3) .