When Rashi uses, what we may losely call, the hononym method, Rashi does not explain new meaning but rather shows an underlying unity in disparate meanings. Rashi will frequently do this by showing an underlying unity in the varied meanings of a Biblical root.

In my article Peshat and Derash found on the world wide web at http://www.Rashiyomi.com/rashi.pdf. I advocate enriching the Rashi explanation using a technique of parallel nifty translations in modern English. Today's examples show this.

The Heberw Biblical root Ayin-Daleth has a fundamental meaning of until. Hence this Biblical root can mean
• hope [ the emotion of waiting unitl success ]
• joy of seeing children grow up (Pr29-17) [ an emotion characterized by the initial work and frustration of raising children until they mature and are productive ]
• marital joys [ this refers not to instant relations but to prolonged relations characterized by an initial maintaining of tension until the joy comes in the end. ]

Applying the above translation to Gn18-12b discussing God's promise to give Sarah a child in her old age we obtain Sarah laughed [in surprise] over the possible pregnancy: "After I have withered I will have the joys [of raising children or of young marital relations] and my husband is old.

Advanced Rashi: We have interpreted the Hebrew Ayin-Daleth-Nun-Hey as referring to drawn out joys where initially one starts with much work and frustration and little joy until one has much joy. Two primary examples of such drawn-out joys are: a) the joys of raising children and b) marital joys. Both these joys are applicable to this verse discussing Sarah giving birth.

Rashi also mentions another possibility: Ayin-Daleth-Nun-Hey refers to the resumption of the menstrual cycle. However the periodic menstrual cycle has little to do with the other meanings of Ayin-Daleth until and joy. I would therefore argue that the menstrual cycle is perceived as a time of abstention from (intimate) marital relations and consequently the reference here would be to the type of marital relations one has in a relationship with alternating periods of abstention and indulgence. I would also argue that the relations in these types of marriages are characterized by until-ness.

One could further argue that the simple meaning of the verse refers to the joy of raising children since the verse speaks about Sarah having a child. Rashi therefore supplements this primary meaning of joy by pointing out that the verse secondarily refers to the joy of marital relations in a situation where there is a cycle of abstention and indulgence; in other words despite her old age Sarah would resume having until relations characterized by length and buildup.