What does the medical abbreviation QS mean?

Quantity Sufficient
As related to a medication order, qs means "quantity sufficient" or inasmuch quantity is sufficient.

It is rarely used anymore, but could be used for medications in which the doctor didn't know the strength or concentration of the medication which would be available/dispensed.
For instance, the physician would give the pharmacist the amount the patient should have (2 mg every eight hours for 14 days) and the pharmacist could dispense the sufficient quantity of whatever strength he had available to fulfill the prescription.

It was also used with fluid balance medications, for instance, potassium. The standing order could be to use potassium in sufficient quantity to maintain a particular blood level.

I've read that qs was previously used to mean "as often as needed" or as needed, but this is now stated as "PRN", usually with a time frame, as in "q 4-6 hours PRN" possibly to avoid overdose!

Hope this helps!