I used this during the Weekly Roundup, so in case you were curious:

osananajimi [oh-saw-nah-nah-jee-me]
幼馴染 (sometimes spelled 幼なじみ)
noun.
(1) A childhood friend

(2) An annoyingly difficult word to say and spell.  It has the same number of “na"s as "banana,” but like banana, you’ll never quite know where to end it. 

(3) The right angle of a love triangle, and/or one half of an unrequited love story, and/or wuv, twoo wuv, which wiw fowwow you fowevah!

Simply put, in anime, if an osanajimi osananajimi shows up, there’s a 98% chance things are ‘bout to get RULL shippy. (This is slightly less true if the osanananajimi is the same sex as the main character, but only slightly.) 

Sometimes they are the main character’s old crush and this is their second chance at romance. Sometimes they’re the platonic bestie who just can’t seem to get the MC to notice them. And sometimes they just drop in for a couple episodes to make life hard on the MC and his/her new OTP.

Whatever the case, adding an osananajimi is an effective (and sometimes lazy) way to inject some backstory and conflict into a series… er, assuming the MC doesn’t just trail off into the old Batman theme song whenever s/he tries to talk about them.

Osanananananananana JIMI! ♪

(Yep. That’s in your head now. You’re welcome.)