I’ve heard this many times, he was doing something on so-and-so’s dime, ie he was doing it under such a person’s aegis or at such a person’s request (I believe that’s the sense). I presume this must have reference to a phone call, as in to drop a dime on someone, to inform on them.

OED has dimedropper, informant, with a first cite of 1966, but I can’t find the on my dime construction. I presume they’re both roughly of the same vintage, alluding to a time when phone calls cost a dime. Or does that cover too large a period to make that a safe assumption? Does anyone, either with access to RHDAS or greater search powers in OED, know which phrase is the elder?

No luck here. It’s not in HDAS. Searching the historical NY Times turns up over 16,000 hits; the few I checked were false hits.

My experience of the expression is different. Unless context denied it, I would think doing something on someone’s dime would mean they are paying for it.

I’m screwing around on the Internet on my company’s dime.

I’ve never heard of the expression, but if it is to do with doing something on behalf of someone else and being paid for it - as has been suggested - is it some sort of variation on terms used in wartime?
Soldiers in the World Wars would be taking the King’s shilling, if fighting in the British armed forces or seeing Europe on Uncle Sam’s nickel, if in the American military

I agree with happydog—it means someone else is paying for it.  I’m pretty sure it has nothing to do with phone calls, and I would guess the choice of “dime” is arbitrary (or based on euphony).  In fact, now that I think of it, it seems to me I’ve heard the phrase with “nickel” as well.

I can’t find “on [someone’s] dime” in HDAS or the Cassell slang dictionary, although the version with “nickel” (which is the one I’ve usually heard) is in HDAS.

Chapman’s _American Slang_ has “on someone’s dime (or nickel) ... At someone’s expense other than the speaker’s ....” and derives it from the price of a phone call. Seems right to me, except that I think it can refer to the speaker too (e.g., “not on my nickel/dime").

A related usage is: “Your dime, my time.” The meaning is: Even though you’ve paid for the call, my time is limited.

Rich

My company does USB qualification testing, among other things, and when we send a piece of tested equipment back to a customer on their account number with whatever shipper we refer to it as being on their nickel.

I have always taken the saying to have a negative connotation because someone doing something on so-and-so’s dime is basically taking advantage of them. I am just guessing, but I want to say this does have to do with telephone calls, particularly those made by chatty secretaries or other employees “on the companys dime.” It probably came from the idea that if pay-phone calls cost a dime, so do calls made from a regular phone. It could also possibly stem from something else that cost a dime, back when you could go a day on a pocket full of change, and take someone out to lunch “on the company’s dime.” Either way, I want to tie this saying directly to company expenses, because the average person would not feel bad, nor easily be caught doing things on their dimes.