Full to the gunwales
Full to the brim; packed tight.
'Gunwales' is pronounced like 'gunnels' and it is often spelled that way too. That's not surprising, as the word is no longer in everyday use - pretty much the only thing that most of us know about gunwales is that they can be full or packed. Consult any search engine and you'll find plenty of examples of "full to the gunnels". Nevertheless, 'gunnels' really is a misspelling.
An early citation of the word comes from Manners and Household Expenses of England, 1466:
Item, the same day my mastyr paid to Roger Fuller, for tymbre for colers [collars] of the maste, and gonne walles...
That suggests that the gunwales of a ship were its 'gun walls', which is exactly what they were - hence the spelling.
Whenever, as a mere landlubber, I stray into defining maritime terms I inevitably get mail from horny-handed sailors telling me that I've got it all wrong. As a precaution, here's the definition of gunwales from the OED:
Gunwales: The upper edge of a ship's side; in large vessels, the uppermost planking, which covers the timber-heads and reaches from the quarter-deck to the forecastle on either side; in small craft, a piece of timber extending round the top side of the hull.
The expressions 'full to the gunwales' or 'packed to the gunwales' were first used as literal references to heavily loaded ships. 'Gunwales' may have been a 15th century word, but there's no mention of the phrase until the 19th century, as in the Unitarian periodical, The Monthly Repository, 1834:
This is the Island of the Golden Fruit. Look, yonder they come! boats - one, two, three, five, a dozen! all laden up to the gunwales with the juicy balls.
The non-nautical use of the phrase didn't come about until the 20th century. A semi-figurative use was made of the phrase in the advertising for the 1944 Dorothy Lamour film, The Fleet's In:
The Fleet's In... and it's loaded to the gunwales with the funniest, friskiest entertainment.
An example of a properly figurative use, i.e. one set on land rather than aboard ship, comes from The New York Magazine, June 1969:
A popular East Side bar, packed to the gunwales with arch young bankers and panicky, pathetic, ersatz Now girls.
Not much has changed in the banking world in forty years it seems, and it's all a long way from gun walls.