In recent years, idioms have received overwhelming attention in language pedagogy. Experts in the field have sought ways to optimize learning and teaching of these prefabricated language chunks. It is now maintained that the meaning of idioms which was once deemed as arbitrary is somehow "motivated" by their literal, original usage. Referring to the etymology of idioms in order to understand their idiomatic meaning is called "etymological elaboration". This study sought to explore the effect of this technique on learners' comprehension as well as retention of idioms. It appears that this effect can be explained by Dual Coding Theory where the etymological association calls up a mental image of a concrete scene which can be stored alongside the verbal form in memory. In this experiment 50 Iranian advanced students studying English at Bahar Language Institute, Shiraz were divided into two homogeneous groups of experimental and control. The test of the former group comprised a matching task pertaining to the origin of idioms, a multiple-choice task concerning the idiomatic meaning of idioms, a multiple-choice task regarding the idiomatic meaning of idioms and a gap-filling task in which subjects were supposed to provide the missing keyword for each idiom. The control group just had a test including multiple choice as well as gap filling tasks. The matching task was utilized as a treatment to demonstrate the effect of knowing the origin of idioms on the participants' performance on the other tasks. The participants' mean scores were calculated, in both "identify-the-meaning" and "gap-filling" tasks, the mean score of participants provided with the etymological elaboration was more than those who did not receive this treatment, which indicated that etymological elaboration enhanced the learners' comprehension and retention of idioms effectively. (Contains 6 tables.)