Means a small amount of anything. Could refer to anything,
depending on the context in which it is used.
The annual company gifts were pretty slim pickens this year.
when there is a lack of the opposite sex around
"damn dawg it is slim pickens tonight, where are all of the hottiemonjaros, how am I a suppose to get a quality blowjob if there are no females to deliver, DAMN IT!"
"If this party is another one of your slim picken parties I'm going to stab you fosho"