1. What does it mean?
짝녀 shortens 짝사랑하는 여자. It names a female crush, often when the affection is one-sided or has not been discussed openly.
Also written or said as jjangnyeo
2. The nuance
It does not tell you the speaker's gender. A 짝녀 is not automatically a 썸녀: the latter suggests a possibly mutual flirting stage, while a crush can exist entirely on one side.
3. Examples
“짝녀가 발표를 잘해서 진심으로 칭찬했어.”
The girl I have a crush on gave a good presentation, so I sincerely complimented her.
“짝녀가 연애할 생각 없다고 해서 내 마음은 혼자 정리하기로 했어.”
My crush said she wasn't interested in dating, so I decided to work through my feelings myself.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.