Discussion Forum—A Way with Words, a fun radio show and podcast about language

I don't really disagree with RobertB, but I'd use different words.   A "spare" tire is an extra one, one I'm not using to drive on; I keep it with me as an extra in case one of the ones I'm using goes flat.   In that sense, a "spare" dime is one I don't actually need at the moment, so I can afford to part with it for your sake.   The verb "to spare" comes from that meaning.

To spare someone a headache, on the other hand, seems different to me.   I suppose when someone says "spare me!" he means "relieve me" (of some kind of trouble).   I can see a shared meaning between —

Oh, wait, I get it.   When Joe is on duty until midnight, and at midnight I show up to take his place—it's my turn now—we say that I "relieve" him.   I might even say I "relieve him of duty", but that phrase also means to fire him, that is to not let him carry that duty any more.   A "relief pitcher" is a pitcher who comes in to pitch in place of the man who's pitching now.   And when we're talking about "war", to "relieve" an army is to go to their aid when they're being attacked.   In that sense, a spare tire is a "relief" tire.   And when one says "spare me that headache", he means "relieve me of the headache".

I've just looked it up in the Online Etymology Dictionary and it says "relief" and "relieve" come from the same Latin word that gave us "lift"; it originally meant to lift a burden from someone, or to lighten it somewhat.