This is, after cumming
onto a girl (usually her face), to take an index finger and put a line on her forehead with your cum
whilst muttering siimbaaa in manner and voice of Mufasa. From the lion king scene where simba is spread with the red dust stuff.
Gave a girl a simba once, was met with a pretty bewildered laugh.. Luckily she also liked the lion king as a child!