Is it ensure, insure, or assure?
1785, originally slang, perhaps a diminutive from dun "dull gray-brown," the form perhaps influenced by monkey. Or possibly from a familiar form of Duncan (cf. dobbin). The older English word was ass (n.1).
A symbol of the Democratic party, introduced in a series of political cartoons by Thomas Nast during the congressional elections of 1874. (Compare elephant.)