Broad in the beam
Having wide hips or buttocks.
This phrase derives from the nautical term 'beam' - the widest point of a ship. Beam is first recorded in Captain John Smith's invaluable record of early seafaring terms - The Seaman's Grammar, 1627:
"Suppose a Ship of 300. Tunnes be 29 foot at the Beame."
The figurative use of beam referring to people's hips came into being in the 20th century. An early citation of that comes in Hugh Walpole's Hans Frost, 1929:
"He stood watching disgustedly Bigges' broad beam."
In the 20th century the term was been adapted to mean 'obese; having wide buttocks'. Presumably this metaphor was aided by the similarity of sound to 'broad in the bum'. Sir Hugh Seymour Walpole used the term 'broad beam' in this context in his 1929 novel Hans Frost:
He stood watching disgustedly Bigges' broad beam.
See other Nautical Phrases.