What does SS stand for when an opponent is missing?

To directly answer your question, 'ss' is a shorter version of 'miss'. In certain regions, they differentiate between this and "MIA", which is meant to refer to a specific opponent whose location is not known rather than a lane with missing people.

From a forum post:

SS (ss bot, ss bot 2, etc...)

Miss, the enemy/enemies are missing from lanes. Normally only called during the laning phase of a game. The lane from where the enemy is missing can be (and should be) said after ss, and a number of enemies missing can be inserted after that; So "ss top 2" simply means that 2 enemy champions that were laning top are now missing, and your teams mid should be extra careful (as should you).

MIA (mia noct, mia shaco, etc...)

Missing in action, an enemy champion is missing in sense that your team does not know his location. For when you are going into a teamfight and only see 4 enemies ahead of you there is a good chance that Fiddlesticks is going to jump to you soon from his hiding place, for example.

On North American servers, people tend to use mia for both purposes:

• mia mid
• mia shaco

The only explanation I've run across is that people outside of North America use SS as an abbreviation for "miSSing". And in N. America, MIA (Missing In Action) is the more common version.

These answers are somewhat incorrect. 'SS' means 'Stay Safe' as opponent left lane he was on/ or is missing from that lane. This originated from DotA, as DotA players started playing LoL in it's early stages. (DotA is predecessor of LoL, and it is a mod for Warcraft III game )