Go with what makes more sense; some search engines are using stemming which reduces expression to its word stem - panty, panties = panti
Most ofter the 'The Porter Stemming Algorithm' is used, here you can check out some examples:
http://maya.cs.depaul.edu/~classes/ds575/porter.html
There is a discussion going on how much google has implemented stemming already - you will see arguments for it and against it. My advice is include the keywords but make it as attractive for the surfer as possible.
Here the Porter Stemming Algo in more detail:
http://www.tartarus.org/~martin/PorterStemmer/