"reduce from the rank or position of a capital city," 1870; see de- + capital (n.1) + -ize. As "to remove the financial capital from," by 1913, from capital (n.2). In reference to letters, "to change from upper case to lower case," by 1899. Related: Decapitalized; decapitalization.
Decalogue
Decameron
decamp
decant
decanter
decapitalize
decapitate
decapitation
decapod
decathlon
decay