Why do we say "Snug as a bug in a rug"?

Meaning:

To feel very comfortable/cozy.

Background:

Sources generally assume this alludes to a clothes-moth cocoon happily ensconced in a cozy rug.

Apparently, the first known reference to this phrase appears in a 1769 play staged by British actor David Garrick in celebration of Shakespeare:

If she has the mopus's (money), I'll have her, as snug as a bug in a rug.

Miriam Webster opines that the expression was likely in common use at the time as it would be unusual to use an unknown metaphor in a play. While the expression may well have been in common use, the logic for reaching that conclusion seems suspect given how many of today's popular phrases appear to have originated in Shakespeare's plays--so it is unclear why introducing a new expression to his audience would be an issue for Garrick writing 150 or so years or so after Shakespeare's time.

Some sources attribute the expression to Benjamin Franklin, but his letter to Georgiana Shipley commiserating her on the death of her pet named Skugg (thought to be a squirrel) came three years later in 1772; so possibly the first known use in the USA, but not overall.

Here Skugg
Lies snug
As a bug
In a rug.

The term snug is of nautical origin, used to describe a trim and compact ship, one well prepared for a voyage or to ride out a storm. Danie Defoe used the term in Robinson Crusoe (1719):

… the wind increased, and we had all hands at work to strike our top-masts, and make every thing snug and close, that the ship might ride as easy as possible.

By the time Defoe wrote this, snug had already migrated into general use to describe a person who was neat and tidy in appearance, or clothing that fitted closely but comfortably; and also to describe being warm and comfortable--whether a human tucked up on a winter's evening or a clothes-moth cocoon ensconced in a rug. Talking of clothes moths, according to an August 2019 article in The Guardian, they are on the increase due to a perfect storm of factors including warmer winters and colder washing cycles.

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