look up any word, like hipster:
 
1.
A commonly-used term in romantic anime, an osananajimi is a friend from infancy, a childhood friend or old playmate with whom one shares the special and intimate understanding that (according to such stories) can only come from having known each other since earliest childhood.
A good example of Osananajimi is Manami Tamura from Ore no imouto
by Edgar4815 November 08, 2010