A combination of the two word parts: 'pseudo', meaning 'like' or 'the same as', and 'kin', meaning 'family'. In reference to your best friend, especially in a mixed gender relationship. A showing of friendly love or affection without the hint of romantic attraction that other words such as 'love' portray. To transliterate, it means 'like/as family'. It is what you substitute for sister and brother, but meaning the same, sans blood relation.
"Hey, pseudokin, what's shakin'?"
Sincerely yours, -pseudokin