Meaning of “whip into a lather”

Question

In New York, basketball phenom Jeremy Lin continues to whip fans of the Knicks into a lather.

What does whip [somebody] into a lather really mean?

Answer 1

It's from whipping a horse to make it gallop faster. Taken to extremes, the horse sweats so much it looks as if it's covered in soap lather. They can and do die from such treatment. OP's context is slightly unusual - a more common form here would be whip into a frenzy (the fans aren't being flogged to the point of exhaustion - they're becoming feverishly excited, like a crazed mob).

I recently saw what at first seemed ridiculous exaggeration in the movie Bite the Bullet (1975). I thought the foam was way over the top, but my horse-riding sister told me it was credible (the movie involves a 700-mile "Endurance Race", so the horses would have been ridden pretty hard).

Thanks to @Hugo for pointing out that this somewhat unusual phenomenon occurs because horse-sweat is rich in latherin, a surfactant which encourages foaming. It can look extreme, but as this comment on the movie at amazon.com, says "Hopefully there was a lot of soap lather being used." I'll second that!

Answer 2

Several dictionaries include a definition of lather that refers to horse sweat. For example, from Merriam-Webster:

1 a : a foam or froth formed when a detergent (as soap) is agitated in water
1 b : foam or froth from profuse sweating (as on a horse)
2 : an agitated or overwrought state : dither (worked himself into a lather)

Here's an example from a 1793 Sporting Magazine:

By no means be induced to countenance those degrading flops tea, coffee, or chocolate, but prove the strength of your stomach by the circumference of a buttock of beef, mollify the glans of the thorax with a jug of strong beer, and prevent any effervescent irritability by the friendly interposition of a bumper (or two) of brandy. Thus internally defended, you stand well prepared to " mount your fiery pegasus," then give the first proof of your prudence in setting out late, that you may enjoy the pleasure of riding hard fifteen or twenty miles, to bring your horse up to the company in a lather, just as the hounds are going to throw off.

I also found several veterinary descriptions of horse sweat looking like a lather. Not only that, there's even a horse-sweat protein called latherin. For example from Equine exercise physiology by David Marlin and K. J. Nankervis:

In addition to high levels of electrolytes, horse sweat contains a protein called latherin, which produces a lather on the skin. Latherin spreads sweat along the surface area over which sweat can evaporate. The latherin content of sweat seems to decrease during prolonged sweating.