Definition - What does Fertile Myrtle mean?
Fertile myrtle is the slang term that is generally used to refer to a woman who appears to be very fertile based either on how quickly she is able to pregnant or the number of times she has been pregnant.
FertilitySmarts explains Fertile Myrtle
This old-fashioned term has the unique ability to elicit strong opinions, especially from within the trying to conceive (TTC) community.
Referring to another individual as a fertile myrtle has the potential to be a misguided comment because there might be more to the appearance of fertility than you are aware. In addition, even if it is an accurate descriptor, reproduction is still a generally considered a personal topic that can make individuals feel awkward, particularly when raised by an acquaintance (or stranger!) or in a public setting.
Along the same lines, referring to yourself as a fertile myrtle has the distinct ability to be annoying to others around you who may have not had a similar experience. Listening to another person speak of their robust fertility can be hard to take when you are experiencing the opposite. Fertility issues are common enough that the odds of such a situation lean more to the side of likely than not likely.
And yes, to others it is simply another saying just like the bees knees or lucky duck; a playful word that shouldn't bring about such a strong case of the feels.