Sexual intercourse with someone outside the primary relationship. The person is strange or unique to the primary relationship.
A usage of this can be seen in the song by Kris Kristofferson's - "Gettin' By, High And Strange"
Soon as I was better I was movin' on gettin' it together gettin' good and gone
And by (gettin' by) high (good and high) and strange (gonna get some strange)