thing·a·ma·jig also thing·um·a·jig (th?ng??-m?-j?g?)n. Informal Something whose name is unknown or forgotten.[Alteration of obsolete thingum (from thing) + jig.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.thing?a?ma?jig or thing?u?ma?jig (???? ? m??d??g) n. a thing for which the speaker does not know or has forgotten the name. [1870?75; thingum or thingummy (based on thing1) + jig1] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.