Someone who hides their (usually large) pain via comedy, outgoingness, or being extroverted. I.E. Someone who copes by making people laugh. Dancing in the rain can be used as an expression to say that someone is happy when bad things are happening around them.
A sand dancer is a wee skinny thing who likes doing skills on the park and doesn’t like to put in a challenge as they are usually shit scared of being hurt by defenders
“Aidan Mcleary is a weesand dancer and doesny want it”
Backup dancer came up about 2014 when I kept having friends bring these randoms out them. These people tend to not know they are backup dancers. They generally bring nothing to the table, occasionally they have money but 9 times they are just here to make you look good. They are sometimes mistaken for acquaintances but normally you don't know much about they. After a while they graduate from backup dancing to actually friends.