As Dex said, “it’s believed”.
There was a thread about the origin of the phrase here: http://boards.straightdope.com/sdmb/showthread.php?t=63738
As noted by samclem (before he became a moderator), it’s uncertain as to whether mercury was being used in hatmaking in England before the phrase first appeared in print in 1837.
According to Snopes that etymology is arguable.
In college I did some research on the classical and medieval uses of common metals. Unfortunately my cites are long lost, but I found this information in the campus library, so it can’t be that obscure.
Aside from alchemy, where it was believed for a long time to be a necessary ingredient to “activate” an alchemical reaction, Mercury was more practically used because of its unique physical properties at room temperature. Royalty and the very elite actually had access to large quantities, and coveted it as a symbol of wealth.
They made “waterbeds” out of it, floating blankets and furs atop a pool of it.
Some cultures traded small vials of it, similar to coinage.
And (in Rome, IIRC) they injested metallic mercury as a laxative. Cite.
My father-in-law swears that in college he and his friends would occasionally drom some mercury into the beer of unsuspecting victims. The drops of mercury would make its way through the digestive tract, and end up in the victims shorts, in, er, very short order. [/iffy anectodal information]
Mercury has a very low viscosity. If you try to stir a beaker full of mercury using a normal stirring rod, you will feel very little resistance and the mercury will hardly move. I suspect if you tried to swim in a pool of mercury you wouldn’t get anywhere.