After all, for a seaman, to scrape the bottom of the thing that supposed to float all the time under his care is the unpardonable sin. No one may know of it, but you never foret the thump - eh? A blow on the very hear.

This exert describes the deep unconquerable hurt a seamen undergoes when he wrecks his ship. This could very easily be used as a metaphor for any situation in which one has carelessly damaged something most valuable to them. For even if no one knows of the inncident, one will carry the memory of their doing the accident in the back of their mind forever.