1570s, from Latin indecorum, noun use of neuter of adjective indecorus "unbecoming, unseemly, unsightly" (see indecorous).
Entries linking to indecorum
indecorous adj.
1680s, "in bad taste," from Latin indecorus "unbecoming, unseemly, unsightly; disgraceful," from in- "not, opposite of, without" (see in- (1)) + decorus "becoming, fitting, seemly, proper" (see decorous). Related: Indecorously; indecorousness (1670s).