"covering for the foot and lower leg," early 14c., from Old French bote "boot" (12c.), with corresponding words in Provençal, Spanish, and Medieval Latin, all of unknown origin, perhaps from a Germanic source. Originally of riding boots only.
From c. 1600 as "fixed external step of a coach." This later was extended to "low outside compartment used for stowing luggage" (1781) and hence the transferred use in Britain in reference to the storage compartment in a motor vehicle (American English uses trunk (n.1)).
Boot-black "person who shines boots and shoes" is from 1817; boot-jack "implement to hold a boot by the heel while the foot is drawn from it" is from 1793. Boot Hill, U.S. frontier slang for "cemetery" (1893, in a Texas panhandle context) probably is an allusion to dying with one's boots on. An old Dorsetshire word for "half-boots" was skilty-boots [Halliwell, Wright].