neat-looking but promiscuous woman
Slang, often derogatory. Describes a woman who appears prim, proper, or innocent but is sexually promiscuous. The term plays on the contrast between a 'seiso-kei' (neat/refined type) appearance and 'bitch' behavior.
There's a rumor that girl is a neat-looking but promiscuous type.
A 'seiso-kei bitch' has a big gap between her appearance and reality.
Compound of 清楚系 (seiso-kei, 'neat/refined type') and ビッチ (bitchi, from English 'bitch'). A modern slang coinage, likely from internet or youth culture, describing a woman whose prim appearance belies promiscuous behavior.