@Nazareno Urban dictionary says, "to brag or show off" (possibly derived from flexing your muscles to show them off).

So yeah,

Children are indeed a heritage from the LORD,
and the fruit of the womb is His reward.
Like arrows in the hand of a warrior,
so are children born in one’s youth.
Blessed is the man
whose quiver is full of them.
He will not be put to shame
when he confronts the enemies at the gate.