I believe you are referring to a word from the book The Hiding
Place by Corrie ten Boom. Best I can tell it must mean something
like fornication or sexual sin. I have been looking online but
can't seem to find answers that agree. Good Luck!
The German language often compounds words. SexSin = sexual sin. Sexsรผnde is knowledge of sexual: shame/crime/sin too heavy a for the immature/innocent to learn of before due time.