The phrase in question refers to a specific type of garment, typically a sleep shirt or pajama top, that features wording or a design referencing the left side of the wearer, implying a preferred sleeping position. An example might be a shirt with the words “Good Night” printed on the left side, meant to be read as the wearer rolls over in bed.
Such apparel can serve several purposes. It can be a humorous gift, expressing a preference for one side of the bed over another. It could also promote comfortable sleep, associating a specific side with relaxation. Furthermore, this type of item may stem from a cultural or familial tradition of acknowledging a favored sleeping posture.