Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
The picture attached to "that flieth" is "to see, to speak, becoming nailed to".
This is a solemn reminder that our words and the words of others are arrows.
They can be poisonous arrows or they can be arrows whose tips are bathed in love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self control.
The psalmist reminds us that we need not fear the poisonous words that fly our way.
However, we can choose to make our words a blessing to everyone we see and speak to.