Refers to cosplays of the Kantai Collection character Sagiri.
Many of these cosplays have actually been with her Kai swimsuit CG (post #5505179), where she wears a white frilled casual one-piece highleg swimsuit with a criss-cross halter, tied on the sides and held with small anchors. Optional are a white choker, a hairband adorned with white flowers, a pair of earrings apparently shaped like lemon slices, a long white shawl, and open toe shoes. Her base swimsuit CG (post #5507429) adds a white frilled turtleneck crop top also tied on the sides which she therefore does technically lose upon becoming Kai. The apparent skimpiness of the swimsuit has in some way provided a minor popularity boost to one of the more forgotten Ayanami sisters and has resulted also in the growing number of other characters wearing it, thus giving it the rei no mizugi (例の水着; i.e. "that swimsuit") tag.
This tag automatically adds sagiri_(kancolle) and cosplay (learn more).