It means Share for Share - you share their pictures, they will share yours.
One common meaning for it is "shoutout for shoutout".
S4S means surfaced on all 4 sides.
Square all 4 sides.
Eased 4 Edges S4S = Surfaced 4 Sides
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If its on facebook pages, it means Share4Share
It means support for support.
If you mean slang as in internet slang, then it means "Certified Old Fart".
Garrett mean in cockney slang
Slang is slang It's the s' "LANG " uage.
In old slang, milkshake mean butt.
It is slang for having sex.
he means what does being a sponge mean in slang