相棒 (aibou) is a word I learned long ago from a J-E dictionary. It means "partner" (or "accomplice") and it's one of the words that the robot dog Aibo's name is supposed to evoke*.

The first kanji, 相 means "together" or "mutual", which makes sense; the second, 棒, means "pole", which doesn't, but I always figured it was ateji for the chummy "person, guy" bou you see in words like 風来坊 (fuuraibou, "come-with-the-wind guy", drifter) and 吝ん坊 (shiwanbou, "stingy guy", skinflint).

This, of course, might be the same bou as the one in words like 坊や (bouya, boy) and 坊ちゃん (botchan, "young master"), and there might well be some connection between all these bous and the word 坊主 (bouzu, monk).

But I digress. The point is, I was wrong. 相棒 isn't ateji: it really is supposed to read "mutual pole [person]". This is because the word 相棒 originally referred to the guy at the other end of the palanquin (etc.) pole you were holding, with whom you obviously needed a pretty good rapport. Hence the metaphorical and in these sadly palanquin-free times far more common meaning of "partner" that I mentioned right at the top.

That's what I get for (a) assuming, and (b) using half-assed J-E dictionaries, I guess. Well, at least I've given up one of those vices.

* The other ones are "AI [artificial intelligence] + bo(t)" and "eye + bo(t)", just for the record.