An extension of the popular NSFW (Not Safe For Work) tag that is commonly used to indicate adult or controversial content that is inappropriate for work environments. The extra O & K represent Or & Kids respectively. It's used as an extended humourous version including kids, another sensitive topic.

NSFWOK : Not Suitable For Work/Wives Or Kids

Used in the subject line to warn that the content of an email or facebook post etc. is meant for a non-wife, non-husband, non Significant Other, Adult ONLY.
Not for Wives, Husbands, SO's or Children.