"Happy as a sandboy" is obviously a simile for happy / content. Where does it come from and what is the eponymous "sandboy"?
Sandboys were boys or men who drove donkeys selling sand, to put down on the floor. Here's W. Meyler's 1805 poem The Rider and Sand-boy: A Tale to paint a picture of the trade.
The earliest printed citation The Phrase Finder found is Pierce Egan's Real Life in London, 1821:
I couldn't find anything earlier, but even better, I found a definition from the same period, in a John Badcock's 1823 Slang, a Dictionary of the Turf, the Ring, the Chase, etc.
Bonus Victorian speculation!
The phrase was also the subject of speculation in the 19th century journal Notes and Queries (a bit like a Victorian Stack Exchange).
A reply to a query about other obsolete phrases in 3rd Series, Volume XI, June 1, 1867:
In 1888, it's noted:
Here's the first 1866 of these unsatisfactory answers:
And here's the second 1870 unsatisfactory answer:
Charles Dickens' The Old Curiousity Shop was published in 1840 but we have earlier citations of the phrase (and a dictionary definition, no less). It's probably safe to say Dickens took the name from the phrase. Our correspondent believes the jolliness is from being drunk ("potations pottle deep"), rather than the earlier suggestions.
World Wide Words says the sandboys' hard work lasted some 15 hours a days for bad pay:
The phrases.co.uk syllabus on this expression does not really give a reason for why sandboys were happy, except that they were intoxicated. However, it does clearly state that the sandboys were young men delivering sand, which would then be spread on floors to absorb liquids (like sawdust).
My aunty who is ninety-one years old told me that the saying came from England where the young boys would carry a bucket of sand and their job was to scrub and leave sand on the steps of buildings so that they weren't slippery. The sand boys would whistle and were happy in their work.