"It's simply an alternate form of 'white leather', and is usually spelled as one word ('whitleather'). The form 'whitleather' dates back to the mid-14th century in its literal sense, and has been used figuratively since the early 17th century.
'Whitleather', as it turns out, is leather, often goatskin, that has been tanned and treated with alum and salt. That process not only lightens the color of the leather, but also renders it soft and pliable, yet very strong and tough, making it a popular material for straps and thongs. Whitleather also used to be known as 'alum leather' and 'Hungarian leather', and a tanner who made whitleather was known as a 'whittawer', the archaic verb 'to taw' meaning (what else?) 'to prepare leather by steeping in alum and salt.' I'm not a big leather buff (I've been wearing the same belt for nearly 20 years, in fact), but I'll bet there's an easier way to make white leather today.
The phrase 'tough as whitleather' in a figurative sense meaning 'tough, hardened, resilient', often applied to a person, has been popular since the 17th century ('A widow o forty-five, As has sludged like a horse all her life, Till 'er's tough as whit-leather..,' D.H. Lawrence, 1913). But 'whitleather' has also been used, figuratively, in comparisons of softness and even paleness ('Her eyes grew preternaturally pale, and her lips wan as whit-leather', 1839).
Interestingly, another use of the word 'whitleather' since the 18th century has been as a synonym for the 'paxwax', the tough, thick ligament connecting the skull of a large quadruped (horse, ox, etc.) to its spine, thus supporting its head. This use of 'whitleather' is probably derived from the ligament's similarity to strong thongs made of 'whitleather'."
http://www.word-detective.com/2010/04/whitleather-tough-as/
