1530s, "proper to one's station or rank," also "tasteful, proper with regard to modesty or social standards," from French décent, or directly from Latin decentem (nominative decens) "becoming, seemly, fitting, proper," present participle of decere "to be fitting or suitable" (from PIE *deke-, from root *dek- "to take, accept"). Related: Decently.
Meaning "kind, pleasant" is from 1902. Meaning "moderate, respectable, good enough" is by 1711. Are you decent? "are you dressed?" (1949) was originally backstage theater jargon.