Where I come from this is a ring piece, or bum hole.
A friend of mine had been playing rugby against some americans and a large chap lifted him up by his ass cheek. He ended up needing surgery after the incident as "he f@%$ing split my gink wide open!" (his words not mine)
A crabby, bitter and unattractive old person. Ginks tend to be impoverished, indignant urban dwellers with the same political and social leanings that they had in their 20s, who blame everyone but themselves for their myriad problems, and accost total strangers to air complaints.
Political demonstrations always seem to include at least one wrinkly, unsmiling, badly-dressed septuagenarian gink sporting a homemade sign and haranguing passersby.