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Two bestest friends who have everything in common and are so alike that they consider themselves as twins, hence the colloquial 'twinnay'.
The term can be used in third person context although can also be addressed directly to the other person.
'That's Tara, she's my twinnay!'
'Heyy twinnay, how's it going?!'
by Spudsasaurus November 01, 2011