It is a recollection of past sexual encounters from a females perspective; can also refer to thoughts of penises from varying racial and ethnic groups.
Lizzie tells Yessi, " Yessi I am having penisnostalgia". Yessi says, " Which dick are you talking about it?". Lizzie says, " I'm recalling the Chinese cock I fucked 5 years ago, the African dude that I banged 2 years ago, and the Puerto Rican dude that bend me over me last night". Yessi says, " Damn that penisnostalgia".