like on who on a lonesome road doth walk in fear and dread, and having once turned round walks on, and no more turns his head, because he knows a frightful fiend doth close behind him tread
It comes from Samuel Taylor Coleridge's "The Rime of the Ancient Mariner,"
Part VI, lines 37-42:
Like one that on a lonesome road
Doth walk in fear and dread,
And having once turned round walks on,
And turns no more his head,
Because he knows a frightful fiend
Doth close behind him tread.
Inspired by it, I'm sure is Housman's:
Oh never fear, man, nought's to dread,
Look not left nor right.
In all the endless road you tread
There's nothing but the night.