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1.
Gang term - person or persons (usualy friends or people from another gang) that are called to help in gang wars/fights

They are called this because they are usaualy phoned just before or during a fight so they arive late and catch the rival gangs by surprize
A Famous case of this was in the fitzroy gardens when The Fitzroy Boyz were fighting SnK and the fitzroy boyz were getting smashed then the ring ins turned up with macchetees, samaurias and knives. A member from SnK had is arm chopped off and then they ran this resulted in a massive change of turf in melbourne.
by hayden_669 September 07, 2006
 
2.
someone who just appears to be there without invitation or invites them self sometimes in an unwanted situation but it is possible to ring in with friends if you see them down the street thus accompaning them but generally ringing in seeing as you were not invited
"hey can i ring in with you guys"
can also be :
ringing in "that dude is always ringing in"
rings in " fred always just rings in "

kinda like gate crashing but not in more of an everyday life sence
by kirsty * January 07, 2006
 
3.
Normally a sporting term but can be appled to any team situation.

Someone, not a regular member of a team, who has been surreptitiously brought in for one match because they have superior skills to the person who normally plays in that position.

In horse racing, A fast horse disguised as, and racing under the name of a known slow horse. So those involved can bet at high odds and get a large winning payout from the bookmakers
Lookout, they're playing a ring in at full back

One of the biggest scandals in Australian horse racing was the Fine Cotton ring in
by BT4a November 04, 2011