Also a cheesy personal ad acronym which stands for In Search Of - It's often used to delineate the this-is-who-I-am section of the ad from the this-is-who-I-want section (for ads so structured, anyway).

I often see it terribly misused (perusing the personals ads?? who? ME?????) as in "GWM ISO of TSG" -- did you catch the redundant 'of' ??