Yours, Etc.: Origins and Uses of 8 Sign-Offs
1570s, echoic (originally of trumpets and birds), akin to or from Latin clangere "resound, ring," and Greek klange "sharp sound," from PIE *klang-, nasalized form of root *kleg- "to cry, sound." Related: Clanged; clanging.
1590s, from clang (v.).