As snug as a bug in a rug… As dead as a doornail…
Many English expressions don’t always make real sense… they are made up of sonorities and are easy to remember for that reason.
They are also totally, TOTALLY, untranslatable !
I mean, you can express their meaning in any other way but word to word will never make sense in another language.
Let’s try :
“Aussi douillet qu’un insecte dans un tapis” or “Aussi mort qu’une clou de porte” – the verbatim translations – would probably raise French eyebrows big time 😉