When someone from the opposing team is missing I have noticed people writing "SS bot/mid/top" to indicate to the rest of the team that they don't have visual on one or more of the people in there lain. But what dose the SS actually stand for?
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
It does stand for a missing champion but I prefer the MIA usage. SS could also potentially stand for summoner spells and I have seen people use that in game.
For example, "mid no ss" clearly means middle lane has no flash or ignite. I have seen people abbreviate this to "mid ss". So, if someone calls this and their opposing laner is still in the lane it means they have just used their summoners, most likely.
SS = Missing or switching sides. It is not used with any other meaning (at least in EUW servers), though I have heard of some Americans using it for summoner spells. I wouldn't recommend that usage; for summoner spells it is more usefull to call the spell they burned, since ignite and flash are way diffrent things.
"Mid no flash"
"Galio burned r"
"ss Mid" - "care bot" (also it is better to not say things you can just ping. Just ping enemy missing (drag to the left and you're fine ;) (also ping danger in the direction they were heading))
I always thought of it meaning single-sided. Usually players tend to call ss when they notice in a dual players lane (usually top or bottom lane), that the one of the two opponents left that lane.
Of course, other interpretations as stated here are also valid. When I started playing LoL back in the US servers, players relied more on calling MIA. Now in the EU servers, it seems that ss is more used.