Leather, of which boots are traditionally made, is notably strong and resistant to wear and tear. As tough as leather was in fact the earliest version of this phrase, although it has now been superseded by the current form.
1967 - Listener - This is no sweet old dolly. She is tough as old boots, working for a living.
Related Idioms and Phrases :
boots and all
Completely - Australian & New Zealand informal
1947 - D. M. Davin - The Rest of Our Lives – The next thing he'll do is counter-attack, boots and all.
the boot is on the other foot
The situation has reversed.
A North American variant is the shoe is on the other foot.
die with your boots on
Die while actively occupied
Die with your boots on was apparently first used in the late 19th century of the deaths of cowboys and others in the American West who were killed in gun battles or hanged.
get the boot
Be dismissed from your job or position - informal
Get the boot comes from the idea of being literally kicked out, as does give someone the boot. A facetious expansion of this idiom is get the Order of the Boot.
hang up your boots
Retire – informal
Boots are seen in this expression as part of a person's working clothes. A common Canadian variant is hang up your skates.
1997 - Farmers Weekly - The hard fact is that all farmers whether the pension scheme is attractive or not are mostly reluctant to hang their boots up.
put the boot in
treat someone brutally especially when they are vulnerable – British informal
The literal sense is kick someone hard when they are already on the ground.
the ability to travel very fast on foot
This phrase comes from the fairy story of Hop o' my Thumb in which magic boots enable the wearer to travel seven leagues at each stride.
Boot here has nothing to do with footwear but comes from an Old English word meaning good, profit or advantage. It survives for the most part only in this phrase and in bootless meaning unavailing or profitless.
1998 - New Scientist - It's an ideal first-year programming book, covering both Java and programming concepts clearly, with humour to boot.
you can bet your boots
you may be absolutely certain - informal
your heart sinks into your boots
used to express a feeling of sudden sadness or dismay
This idiom has given rise to the adjective heartsink used in the medical profession to describe a patient who causes their medical practitioner to experience such a feeling usually as a result of making frequent visits to the surgery to complain of persistent but unidentifiable ailments.